View source for Polyhedron

{{short description|Three-dimensional shape with flat faces, straight edges, and sharp corners}}
{{other uses}}
{{redirect-distinguish|Polyhedra|Polyhedra (software)}}
{{infobox
 | name = Polyhedron
 | title = Examples of polyhedra
 | image = {{multiple image|border=infobox|perrow=2/2/2|total_width=350
 | image1 = Tetrahedron.jpg
 | alt1 =
 | caption1 = [[Regular tetrahedron]]<br>([[Platonic solid]])
 | image2 = Small stellated dodecahedron.png
 | alt2 =
 | caption2 = [[Small stellated dodecahedron]]<br>([[Kepler&ndash;Poinsot polyhedron]])
 | image3 = Icosidodecahedron.png
 | alt3 = 
 | caption3 = [[Icosidodecahedron]]<br>([[Archimedean solid]])
 | image4 = Great cubicuboctahedron.png
 | alt4 = 
 | caption4 = [[Great cubicuboctahedron]]<br>([[Uniform star-polyhedron]])
 | image5 = Rhombic triacontahedron.png
 | alt5 =
 | caption5 = [[Rhombic triacontahedron]]<br>([[Catalan solid]])
 | image6 = Hexagonal torus.svg
 | alt6 =
 | caption6 = A [[toroidal polyhedron]]
}}
 | label1 = Definition
 | data1 = A three-dimensional example of the more general [[polytope]] in any number of dimensions
}}

In [[geometry]], a '''polyhedron''' ({{plural form}}: '''polyhedra''' or '''polyhedrons'''; {{ety|el|''[[wikt:πολύς|πολύ]]'' {{nowrap|(poly-)}} |many||''[[wikt:ἕδρα|ἕδρον]]'' {{nowrap|(-hedron)}} |base, seat}}) is a [[three-dimensional figure]] with flat [[polygon]]al [[Face (geometry)|faces]], straight [[Edge (geometry)|edges]] and sharp corners or [[Vertex (geometry)|vertices]].

A ''convex polyhedron'' is a polyhedron that bounds a [[convex set]]. Every convex polyhedron can be constructed as the [[convex hull]] of its vertices, and for every finite set of points, not all on the same plane, the convex hull is a convex polyhedron. [[Cube]]s and [[Pyramid (geometry)|pyramids]] are examples of convex polyhedra.

A polyhedron is a generalization of a 2-dimensional [[polygon]] and a 3-dimensional specialization of a [[polytope]], a more general concept in any number of [[dimension]]s.

{{TOC limit|3}}

==Definition==
[[convex polyhedron|Convex polyhedra]] are well-defined, with several equivalent standard definitions. However, the formal mathematical definition of polyhedra that are not required to be convex has been problematic.
Many definitions of "polyhedron" have been given within particular contexts,<ref name="lakatos">{{citation
 | last = Lakatos | first = Imre | editor2-first = Elie | editor2-last = Zahar | editor1-first = John | editor1-last = Worrall | author-link = Imre Lakatos
 | doi = 10.1017/CBO9781316286425
 | isbn = 978-1-107-53405-6
 | location = Cambridge
 | mr = 3469698
 | orig-date = 1976
 | page = 16
 | publisher = Cambridge University Press
 | quote = definitions are frequently proposed and argued about
 | series = Cambridge Philosophy Classics
 | title = Proofs and Refutations: The logic of mathematical discovery
 | year = 2015| title-link = Proofs and Refutations }}.</ref> some more rigorous than others, and there is no universal agreement over which of these to choose.
Some of these definitions exclude shapes that have often been counted as polyhedra (such as the [[star polyhedron|self-crossing polyhedra]]) or include
shapes that are often not considered as valid polyhedra (such as solids whose boundaries are not [[manifold]]s). As [[Branko Grünbaum]] observed,
{{Blockquote|"The Original Sin in the theory of polyhedra goes back to Euclid, and through Kepler, Poinsot, Cauchy and many others ... at each stage ... the writers failed to define what are the polyhedra".<ref name=sin>{{citation
 | last = Grünbaum | first = Branko | author-link = Branko Grünbaum
 | editor1-last = Bisztriczky | editor1-first = Tibor
 | editor2-last = McMullen | editor2-first = Peter 
 | editor3-last = Schneider|editor3-first = Rolf
 | editor4-last = Weiss | editor4-first = A.
 | contribution = Polyhedra with hollow faces
 | doi = 10.1007/978-94-011-0924-6_3
 | isbn = 978-94-010-4398-4
 | location = Dordrecht
 | mr = 1322057
 | pages = 43–70
 | publisher = Kluwer Acad. Publ.
 | title = Proceedings of the NATO Advanced Study Institute on Polytopes: Abstract, Convex and Computational
 | year = 1994}}; for quote, see p.&nbsp;43.</ref>}}
Nevertheless, there is general agreement that a polyhedron is a solid or surface that can be described by its [[vertex (geometry)|vertices]] (corner points), [[edge (geometry)|edges]] (line segments connecting certain pairs of vertices),
[[face (geometry)|faces]] (two-dimensional [[polygon]]s), and that it sometimes can be said to have a particular three-dimensional interior [[volume]].
One can distinguish among these different definitions according to whether they describe the polyhedron as a solid, whether they describe it as a surface, or whether they describe it more abstractly based on its [[incidence geometry]].<ref>{{citation|contribution=Polyhedra: Surfaces or solids?|first=Arthur L.|last=Loeb|author-link= Arthur Lee Loeb|pages=65–75|title=Shaping Space: Exploring Polyhedra in Nature, Art, and the Geometrical Imagination|edition=2nd|editor-first=Marjorie|editor-last=Senechal|editor-link=Marjorie Senechal|publisher=Springer|year=2013|doi=10.1007/978-0-387-92714-5_5|isbn=978-0-387-92713-8 }}</ref>
* A common and somewhat naive definition of a polyhedron is that it is a solid whose boundary can be covered by finitely many planes<ref>{{citation|title=Solid Geometry|first=Joseph P.|last=McCormack|publisher=D. Appleton-Century Company|year=1931|page=416}}.</ref><ref>{{citation|title=Computational Geometry: Algorithms and Applications|last1=de Berg|first1=M.|author1-link= Mark de Berg |last2=van Kreveld|first2=M.|author2-link= Marc van Kreveld |last3=Overmars|first3=M.|author3-link=Mark Overmars|last4=Schwarzkopf|first4=O.|author4-link=Otfried Cheong|edition=2nd|publisher=Springer|year=2000|page=64}}.</ref> or that it is a solid formed as the union of finitely many convex polyhedra.<ref>{{SpringerEOM|title=Polyhedron, abstract|id=Polyhedron,_abstract&oldid=25452|first=S.V.|last=Matveev}}</ref> Natural refinements of this definition require the solid to be bounded, to have a connected interior, and possibly also to have a connected boundary. The faces of such a polyhedron can be defined as the [[Connected space|connected components]] of the parts of the boundary within each of the planes that cover it, and the edges and vertices as the line segments and points where the faces meet. However, the polyhedra defined in this way do not include the self-crossing star polyhedra, whose faces may not form [[simple polygon]]s, and some of whose edges may belong to more than two faces.<ref>{{citation|title=Adventures Among the Toroids: A study of orientable polyhedra with regular faces|title-link= Adventures Among the Toroids |first=B. M.|last=Stewart|author-link= Bonnie Stewart |edition=2nd|year=1980|page=6}}.</ref>
* Definitions based on the idea of a bounding surface rather than a solid are also common.<ref name=cromwell>{{citation
 | last = Cromwell | first = Peter R.
 | isbn = 978-0-521-55432-9
 | location = Cambridge
 | mr = 1458063
 | publisher = Cambridge University Press
 | title = Polyhedra | title-link = Polyhedra (book)
 | year = 1997}}; for definitions of polyhedra, see pp. 206–209; for polyhedra with equal regular faces, see p. 86.</ref> For instance, {{harvtxt|O'Rourke|1993}} defines a polyhedron as a union of [[convex polygon]]s (its faces), arranged in space so that the intersection of any two polygons is a shared vertex or edge or the [[empty set]] and so that their union is a [[manifold]].<ref>{{citation|title=Computational Geometry in C|journal=Computers in Physics|volume=9|issue=1|first=Joseph|last=O'Rourke|author-link=Joseph O'Rourke (professor)|year=1993|pages=113–116|bibcode=1995ComPh...9...55O|doi=10.1063/1.4823371|url=http://www.gbv.de/dms/goettingen/241632501.pdf }}.</ref> If a planar part of such a surface is not itself a convex polygon, O'Rourke requires it to be subdivided into smaller convex polygons, with flat [[dihedral angle]]s between them. Somewhat more generally, Grünbaum defines an ''acoptic polyhedron'' to be a collection of simple polygons that form an embedded manifold, with each vertex incident to at least three edges and each two faces intersecting only in shared vertices and edges of each.<ref name=acoptic>{{citation
 | last = Grünbaum
 | first = Branko
 | author-link = Branko Grünbaum
 | contribution = Acoptic polyhedra
 | doi = 10.1090/conm/223/03137
 | mr = 1661382
 | pages = 163–199
 | publisher = American Mathematical Society
 | location = Providence, Rhode Island
 | series = Contemporary Mathematics
 | title = Advances in discrete and computational geometry (South Hadley, MA, 1996)
 | contribution-url = https://sites.math.washington.edu/~grunbaum/BG225.Acoptic%20polyhedra.pdf
 | volume = 223
 | year = 1999
 | isbn = 978-0-8218-0674-6
 | access-date = 2022-07-01
 | archive-date = 2021-08-30
 | archive-url = https://web.archive.org/web/20210830211936/https://sites.math.washington.edu/~grunbaum/BG225.Acoptic%20polyhedra.pdf
 | url-status = dead
 }}.</ref> Cromwell's ''[[Polyhedra (book)|Polyhedra]]'' gives a similar definition but without the restriction of at least three edges per vertex. Again, this type of definition does not encompass the self-crossing polyhedra.<ref name=cromwell/> Similar notions form the basis of topological definitions of polyhedra, as subdivisions of a topological manifold into [[Disk (mathematics)|topological disks]] (the faces) whose pairwise intersections are required to be points (vertices), topological arcs (edges), or the empty set. However, there exist topological polyhedra (even with all faces triangles) that cannot be realized as acoptic polyhedra.<ref>{{citation
 | last1 = Bokowski | first1 = J.
 | last2 = Guedes de Oliveira | first2 = A.
 | doi = 10.1007/s004540010027
 | issue = 2–3
 | journal = [[Discrete and Computational Geometry]]
 | mr = 1756651
 | pages = 197–208
 | title = On the generation of oriented matroids
 | volume = 24
 | year = 2000| doi-access = free
 }}.</ref>
* One modern approach is based on the theory of [[abstract polyhedron|abstract polyhedra]]. These can be defined as [[partially ordered set]]s whose elements are the vertices, edges, and faces of a polyhedron. A vertex or edge element is less than an edge or face element (in this partial order) when the vertex or edge is part of the edge or face. Additionally, one may include a special bottom element of this partial order (representing the empty set) and a top element representing the whole polyhedron. If the sections of the partial order between elements three levels apart (that is, between each face and the bottom element, and between the top element and each vertex) have the same structure as the abstract representation of a polygon, then these partially ordered sets carry exactly the same information as a topological polyhedron.{{cn|date=May 2023|reason=the [[11-cell]] and [[57-cell]] are valid abstract polytopes but not valid topological polytopes; the latter approach assumes simple balls but the former does not. How can these be said to carry the "same" information?}} However, these requirements are often relaxed, to instead require only that sections between elements two levels apart have the same structure as the abstract representation of a line segment.<ref name="bursta">{{citation
 | last1 = Burgiel | first1 = H.
 | last2 = Stanton | first2 = D.
 | doi = 10.1007/s004540010030
 | issue = 2–3
 | journal = [[Discrete and Computational Geometry]]
 | mr = 1758047
 | pages = 241–255
 | title = Realizations of regular abstract polyhedra of types {3,6} and {6,3}
 | volume = 24
 | year = 2000| doi-access = free
 }}.</ref> (This means that each edge contains two vertices and belongs to two faces, and that each vertex on a face belongs to two edges of that face.) Geometric polyhedra, defined in other ways, can be described abstractly in this way, but it is also possible to use abstract polyhedra as the basis of a definition of geometric polyhedra. A ''realization'' of an abstract polyhedron is generally taken to be a mapping from the vertices of the abstract polyhedron to geometric points, such that the points of each face are coplanar. A geometric polyhedron can then be defined as a realization of an abstract polyhedron.<ref name=grunbaum-same>{{citation | last = Grünbaum | first = Branko | title = Discrete and Computational Geometry: The Goodman–Pollack Festschrift | author-link = Branko Grünbaum | editor1-last = Aronov | editor1-first = Boris | editor1-link = Boris Aronov | editor2-last = Basu | editor2-first = Saugata | editor3-last = Pach | editor3-first = János | editor3-link = János Pach | editor4-last = Sharir | editor4-first = Micha | editor4-link = Micha Sharir | contribution = Are your polyhedra the same as my polyhedra? | contribution-url = https://faculty.washington.edu/moishe/branko/BG249.Your%20polyh-my%20polyh.pdf | doi = 10.1007/978-3-642-55566-4_21 | mr = 2038487 | pages = 461–488 | publisher = Springer | location = Berlin | series = Algorithms and Combinatorics | volume = 25 | year = 2003 | isbn = 978-3-642-62442-1 | citeseerx = 10.1.1.102.755  }}</ref> Realizations that omit the requirement of face planarity, that impose additional requirements of symmetry, or that map the vertices to higher dimensional spaces have also been considered.<ref name="bursta"/> Unlike the solid-based and surface-based definitions, this works perfectly well for star polyhedra. However, without additional restrictions, this definition allows [[Degeneracy (mathematics)|degenerate]] or unfaithful polyhedra (for instance, by mapping all vertices to a single point) and the question of how to constrain realizations to avoid these degeneracies has not been settled.

In all of these definitions, a polyhedron is typically understood as a three-dimensional example<!-- There are polyhedra in 4-dimensions like the [[regular skew polyhedron]] with flat faces and skew [[vertex figure]]s, and seen as an approximation of the surface of a 4D [[duocylinder]].--> of the more general [[polytope]] in any number of dimensions. For example, a polygon has a two-dimensional body and no faces, while a [[4-polytope]] has a four-dimensional body and an additional set of three-dimensional "cells".
However, some of the literature on higher-dimensional geometry uses the term "polyhedron" to mean something else: not a three-dimensional polytope, but a shape that is different from a polytope in some way. For instance, some sources define a convex polyhedron to be the intersection of finitely many [[Half-space (geometry)|half-spaces]], and a polytope to be a bounded polyhedron.<ref name="polytope-bounded-1">{{citation
 | last = Grünbaum | first = Branko | author-link = Branko Grünbaum
 | doi = 10.1007/978-1-4613-0019-9
 | edition = 2nd
 | isbn = 978-0-387-00424-2
 | location = New York
 | mr = 1976856
 | page = 26
 | publisher = Springer-Verlag
 | ref = Grünbaum-Convex-Polytopes<!-- Don't interfere with harv links to the other Grünbaum 2003 reference -->
 | series = Graduate Texts in Mathematics
 | title = Convex Polytopes
 | title-link = Convex Polytopes
 | volume = 221
 | year = 2003}}.</ref><ref name="polytope-bounded-2">{{citation
 | last1 = Bruns | first1 = Winfried
 | last2 = Gubeladze | first2 = Joseph
 | contribution = Definition 1.1
 | contribution-url = https://books.google.com/books?id=pbgg1pFxW8YC&pg=PA5
 | doi = 10.1007/b105283
 | isbn = 978-0-387-76355-2
 | mr = 2508056
 | page = 5
 | publisher = Springer | location = Dordrecht
 | series = Springer Monographs in Mathematics
 | title = Polytopes, Rings, and ''K''-theory
 | year = 2009| citeseerx = 10.1.1.693.2630
 }}.</ref> The remainder of this article considers only three-dimensional polyhedra.

==Convex polyhedra==
{{multiple image
 | total_width = 300
 | align = left
 | perrow = 2
 | image1 = Hexagonal pyramid.png
 | image2 = Afgeknotte driezijdige piramide.png
 | image3 = Triakisicosahedron.jpg
 | image4 = Triaugmented triangular prism (symmetric view).svg
 | footer = Top left to bottom right: [[hexagonal pyramid]] as the family of [[prismatoid]]s, [[truncated tetrahedron]] as the family of [[Archimedean solid]]s, [[triakis icosahedron]] as the family of [[Catalan solid]]s, and [[triaugmented triangular prism]] as the family of both [[deltahedron]]s and [[Johnson solid]]s. All of these classes are convex polyhedrons.
}}
A ''convex polyhedron'' is a polyhedron that forms a [[convex set]] as a solid. That being said, it is a three-dimensional solid whose every line segment connects two of its points lies its [[Interior (topology)|interior]] or on its [[Boundary (topology)|boundary]]; none of its faces are [[coplanar]] (they do not share the same plane) and none of its edges are [[Colinearity|collinear]] (they are not segments of the same line).<ref name="by">{{cite conference|mode=cs2
 | last1 = Boissonnat | first1 = J. D.
 | last2 = Yvinec | first2 = M.
 | date = June 1989
 | title = Probing a scene of non convex polyhedra
 | conference = Proceedings of the fifth annual symposium on Computational geometry
 | pages = 237–246
 | doi = 10.1145/73833.73860
}}.</ref><ref name="litchenberg">{{citation
 | last = Litchenberg | first = D. R.
 | year = 1988
 | title = Pyramids, Prisms, Antiprisms, and Deltahedra
 | journal = The Mathematics Teacher
 | volume = 81
 | issue = 4
 | pages = 261–265
 | jstor = 27965792
}}.</ref> A convex polyhedron can also be defined as a bounded intersection of finitely many [[Half-space (geometry)|half-spaces]], or as the [[convex hull]] of finitely many points, in either case, restricted to intersections or hulls that have nonzero volume.<ref name="polytope-bounded-1"/><ref name="polytope-bounded-2"/>

Important classes of convex polyhedra include the family of [[prismatoid]], the [[Platonic solid]]s, the [[Archimedean polyhedron|Archimedean solids]] and their duals the [[Catalan solid]]s, and the regular polygonal faces polyhedron. The prismatoids are the polyhedron whose vertices lie on two parallel planes and their faces are likely to be trapezoids and triangles.<ref name="prismatoid">{{citation
 | last1 = Kern | first1 = William F.
 | last2 = Bland | first2 = James R.
 | title = Solid Mensuration with proofs
 | url = https://books.google.com/books?id=Y6cAAAAAMAAJ
 | year = 1938
 | page = 75
}}.</ref> Examples of prismatoids are [[Pyramid (geometry)|pyramid]]s, [[Wedge (geometry)|wedge]]s, [[parallelipiped]]s, [[Prism (geometry)|prism]]s, [[antiprism]]s, [[cupola]]s, and [[frustum]]s. The Platonic solids are the five ancientness polyhedrons&mdash;[[regular tetrahedron|tetrahedron]], [[regular octahedron|octahedron]], [[regular icosahedron|icosahedron]], [[cube]], and [[regular dodecahedron|dodecahedron]]&mdash;classified by [[Plato]] in his [[Timaeus (dialogue)|''Timaeus'']] whose connecting four [[classical element]]s of nature.{{sfnp|Cromwell|1997|p=51&ndash;52}} The Archimedean solids are the class of thirteen polyhedrons whose faces are all regular polygons and whose vertices are symmetric to each other;{{efn|The Archimedean solids once had fourteenth solid known as [[pseudorhombicuboctahedron]], mistakenly constructing [[rhombicuboctahedron]]. However, it was debarred for having no [[vertex-transitive]] property, which included it to the Johnson solid instead.<ref name="14th-archimedean">{{citation
 | last = Grünbaum | first = Branko | author-link = Branko Grünbaum
 | doi = 10.4171/EM/120
 | issue = 3
 | journal = [[Elemente der Mathematik]]
 | mr = 2520469
 | pages = 89–101
 | title = An enduring error
 | url = https://digital.lib.washington.edu/dspace/bitstream/handle/1773/4592/An_enduring_error.pdf
 | volume = 64
 | year = 2009| doi-access = free
}}. Reprinted in {{citation|title=The Best Writing on Mathematics 2010|editor-first=Mircea|editor-last=Pitici|publisher=Princeton University Press|year=2011|pages=18–31}}.</ref>}} their dual polyhedrons are [[Catalan solid]]s.<ref name="diudea">{{citation
 | last = Diudea | first = M. V.
 | year = 2018
 | title = Multi-shell Polyhedral Clusters
 | publisher = [[Springer Science+Business Media|Springer]]
 | isbn = 978-3-319-64123-2
 | doi = 10.1007/978-3-319-64123-2
 | url = https://books.google.com/books?id=p_06DwAAQBAJ&pg=PA39
 | page = 39
}}.</ref> The class of regular polygonal faces polyhedron are the [[deltahedron]] (whose faces are all [[equilateral triangle]]s and [[Johnson solid]]s (whose faces are arbitrary regular polygons).<ref name="cundy">{{citation
 | last = Cundy | first = H. Martyn | author-link = Martyn Cundy
 | title = Deltahedra
 | journal = [[Mathematical Gazette]]
 | volume = 36 | pages = 263–266
 | year = 1952
 | doi = 10.2307/3608204
 | jstor = 3608204
}}.</ref><ref name="berman">{{citation
 | last = Berman | first = Martin
 | doi = 10.1016/0016-0032(71)90071-8
 | journal = Journal of the Franklin Institute
 | mr = 290245
 | pages = 329–352
 | title = Regular-faced convex polyhedra
 | volume = 291
 | year = 1971| issue = 5
}}.</ref>

The convex polyhedron can be categorized into [[elementary polyhedron]] or composite polyhedron. An elementary polyhedron is a convex regular-faced polyhedron that cannot be produced into two or more polyhedrons by slicing it with a plane.{{sfnp|Hartshorne|2000|p=[https://books.google.com/books?id=EJCSL9S6la0C&pg=PA464 464]}} Quite opposite to a composite polyhedron, it can be alternatively defined as a polyhedron that can be constructed by attaching more elementary polyhedrons. For example, [[triaugmented triangular prism]] is a composite polyhedron since it can be constructed by attaching three [[equilateral square pyramid]]s onto the square faces of a [[triangular prism]]; the square pyramids and the triangular prism are elementary.<ref name="timofeenko-2010">{{citation
 | last = Timofeenko | first = A. V.
 | year = 2010
 | title = Junction of Non-composite Polyhedra
 | journal = St. Petersburg Mathematical Journal
 | volume = 21 | issue = 3 | pages = 483–512
 | doi = 10.1090/S1061-0022-10-01105-2
 | url = https://www.ams.org/journals/spmj/2010-21-03/S1061-0022-10-01105-2/S1061-0022-10-01105-2.pdf
}}.</ref>

[[File:Midsphere.png|thumb|upright|A canonical polyhedron]]
A [[midsphere]] of a convex polyhedron is a sphere tangent to every edge of a polyhedron, an intermediate sphere in radius between the [[insphere]] and [[circumsphere]], for polyhedra for which all three of these spheres exist. Every convex polyhedron is combinatorially equivalent to a ''canonical polyhedron'', a polyhedron that has a midsphere whose center coincides with the [[centroid]] of the polyhedron. The shape of the canonical polyhedron (but not its scale or position) is uniquely determined by the combinatorial structure of the given polyhedron.<ref>{{citation
 | last = Schramm | first = Oded
 | date = 1992-12-01
 | title = How to cage an egg
 | journal = Inventiones Mathematicae
 | language = en
 | volume = 107 | issue = 1 | pages = 543–560
 | doi = 10.1007/BF01231901
 | bibcode = 1992InMat.107..543S
 | issn = 1432-1297
}}.</ref>

Some polyhedrons do not have the property of convexity, and they are called ''non-convex polyhedrons''. Such polyhedrons are [[star polyhedron]]s and [[Kepler&ndash;Poinsot polyhedron]]s, which constructed by either [[stellation]] (process of extending the faces&mdash;within their planes&mdash;so that they meet) or [[faceting]] (whose process of removing parts of a polyhedron to create new faces&mdash;or facets&mdash;without creating any new vertices).<ref name="bridge">{{citation
 | last = Bridge | first = N.J.
 | title = Facetting the dodecahedron
 | year = 1974
 | journal = Acta Crystallographica
 | volume = A30
 | issue = 4
 | doi = 10.1107/S0567739474001306
 | pages = 548–552| bibcode = 1974AcCrA..30..548B
}}.</ref><ref>{{citation
 | last = Inchbald | first = G.
 | title = Facetting diagrams
 | year = 2006
 | journal = The Mathematical Gazette
 | volume = 90
 | issue = 518
 | pages = 253–261
 | doi = 10.1017/S0025557200179653| s2cid = 233358800
}}.</ref> A facet of a polyhedron is any polygon whose corners are vertices of the polyhedron, and is not a ''[[Face (geometry)|face]]''.<ref name="bridge"/> The stellation and faceting are inverse or reciprocal processes: the dual of some stellation is a faceting of the dual to the original polyhedron.

== Characteristics ==

===Number of faces===
Polyhedra may be classified and are often named according to the number of faces. The naming system is based on Classical Greek, and combines a prefix counting the faces with the suffix "hedron", meaning "base" or "seat" and referring to the faces. For example a [[tetrahedron]] is a polyhedron with four faces, a [[pentahedron]] is a polyhedron with five faces, a [[hexahedron]] is a polyhedron with six faces, etc.<ref>{{citation|title=The New Elements of Mathematics, Volume II: Algebra and Geometry|first=Charles S.|last=Peirce|author-link=Charles Sanders Peirce|editor-first=Carolyn|editor-last=Eisele|editor-link=Carolyn Eisele|year=1976|publisher=Mouton Publishers & Humanities Press|page=297|isbn=9783110818840 |url=https://books.google.com/books?id=Z3ldDwAAQBAJ&pg=PA297}}</ref> For a complete list of the Greek numeral prefixes see {{slink|Numeral prefix|Table of number prefixes in English}}, in the column for Greek cardinal numbers. The names of tetrahedra, hexahedra, octahedra (8-sided polyhedra), dodecahedra (12-sided polyhedra), and icosahedra (20-sided polyhedra) are sometimes used without additional qualification to refer to the [[Platonic solid]]s, and sometimes used to refer more generally to polyhedra with the given number of sides without any assumption of symmetry.<ref>{{citation|orig-date=1996|year=2020|title=Crystal Structures: Patterns and Symmetry|first1=Michael|last1=O'Keefe|first2=Bruce G.|last2=Hyde|publisher=Dover Publications|page=134|isbn=9780486836546 |url=https://books.google.com/books?id=_MjPDwAAQBAJ&pg=PA134}}</ref>

===Topological classification===
{{main|Toroidal polyhedron}}
[[File:Tetrahemihexahedron rotation.gif|thumb|The [[tetrahemihexahedron]], a non-orientable self-intersecting polyhedron with four triangular faces (red) and three square faces (yellow). As with a [[Möbius strip]] or [[Klein bottle]], a continuous path along the surface of this polyhedron can reach the point on the opposite side of the surface from its starting point, making it impossible to separate the surface into an inside and an outside. (Topologically, this polyhedron is a [[real projective plane]].)]]
Some polyhedra have two distinct sides to their surface. For example, the inside and outside of a [[convex polytope|convex polyhedron]] paper model can each be given a different colour (although the inside colour will be hidden from view). These polyhedra are [[Orientability|orientable]]. The same is true for non-convex polyhedra without self-crossings. Some non-convex self-crossing polyhedra can be coloured in the same way but have regions turned "inside out" so that both colours appear on the outside in different places; these are still considered to be orientable. However, for some other self-crossing polyhedra with simple-polygon faces, such as the [[tetrahemihexahedron]], it is not possible to colour the two sides of each face with two different colours so that adjacent faces have consistent colours.
In this case the polyhedron is said to be non-orientable. For polyhedra with self-crossing faces, it may not be clear what it means for adjacent faces to be consistently coloured, but for these polyhedra it is still possible to determine whether it is orientable or non-orientable by considering a topological [[cell complex]] with the same incidences between its vertices, edges, and faces.<ref name=ringel>{{citation
 | last = Ringel | first = Gerhard
 | contribution = Classification of surfaces
 | doi = 10.1007/978-3-642-65759-7_3
 | pages = 34–53
 | publisher = Springer
 | title = Map Color Theorem
 | year = 1974| isbn = 978-3-642-65761-0
 }}</ref>

A more subtle distinction between polyhedron surfaces is given by their [[Euler characteristic]], which combines the numbers of vertices <math>V</math>, edges <math>E</math>, and faces <math>F</math> of a polyhedron into a single number <math>\chi</math> defined by the formula
:<math>\chi=V-E+F.\ </math>
The same formula is also used for the Euler characteristic of other kinds of topological surfaces. It is an invariant of the surface, meaning that when a single surface is subdivided into vertices, edges, and faces in more than one way, the Euler characteristic will be the same for these subdivisions. For a convex polyhedron, or more generally any simply connected polyhedron with surface a topological sphere, it always equals 2.  For more complicated shapes, the Euler characteristic relates to the number of [[toroid]]al holes, handles or [[cross-cap]]s in the surface and will be less than 2.<ref>{{citation
 | last = Richeson | first = David S. | author-link = David Richeson
 | isbn = 978-0-691-12677-7
 | location = Princeton, NJ
 | mr = 2440945
 | publisher = Princeton University Press
 | title = Euler's Gem: The polyhedron formula and the birth of topology
 | title-link = Euler's Gem
 | year = 2008}}, pp. 157, 180.</ref>
All polyhedra with odd-numbered Euler characteristic are non-orientable. A given figure with even Euler characteristic may or may not be orientable. For example, the one-holed [[toroid]] and the [[Klein bottle]] both have <math>\chi = 0</math>, with the first being orientable and the other not.<ref name=ringel/>

For many (but not all) ways of defining polyhedra, the surface of the polyhedron is required to be a [[manifold]]. This means that every edge is part of the boundary of exactly two faces (disallowing shapes like the union of two cubes that meet only along a shared edge) and that every vertex is incident to a single alternating cycle of edges and faces (disallowing shapes like the union of two cubes sharing only a single vertex). For polyhedra defined in these ways, the [[classification of manifolds]] implies that the topological type of the surface is completely determined by the combination of its Euler characteristic and orientability. For example, every polyhedron whose surface is an orientable manifold and whose Euler characteristic is 2 must be a topological sphere.<ref name=ringel/>

A [[toroidal polyhedron]] is a polyhedron whose [[Euler characteristic]] is less than or equal to 0, or equivalently whose [[Genus (mathematics)|genus]] is 1 or greater.  Topologically, the surfaces of such polyhedra are [[torus]] surfaces having one or more holes through the middle.<ref>{{citation|first=B. M.|last=Stewart|title=Adventures Among the Toroids: A Study of Orientable Polyhedra with Regular Faces|title-link= Adventures Among the Toroids |edition=2nd|year=1980|isbn=978-0-686-11936-4|publisher=B. M. Stewart}}.</ref>

===Duality===
{{main|Dual polyhedron}}
[[File:Dual Cube-Octahedron.svg|thumb|180px|The octahedron is dual to the cube]]

For every convex polyhedron, there exists a dual polyhedron having
* faces in place of the original's vertices and vice versa, and
* the same number of edges.
The dual of a convex polyhedron can be obtained by the process of [[Dual polyhedron#Polar reciprocation|polar reciprocation]].<ref>{{citation
 | last1 = Cundy | first1 = H. Martyn | author1-link = Martyn Cundy
 | last2 = Rollett | first2 = A.P.
 | edition = 2nd
 | location = Oxford
 | mr = 0124167
 | publisher = Clarendon Press
 | title = Mathematical models
 | title-link = Mathematical Models (Cundy and Rollett)
 | year = 1961
 | contribution = 3.2 Duality
 | pages = 78–79}}.<!-- Describes only duality by polar reciprocation through the midsphere --></ref>  Dual polyhedra exist in pairs, and the dual of a dual is just the original polyhedron again. Some polyhedra are self-dual, meaning that the dual of the polyhedron is congruent to the original polyhedron.<ref>{{citation
 | last1 = Grünbaum
 | first1 = B.
 | author1-link = Branko Grünbaum
 | last2 = Shephard
 | first2 = G.C.
 | author2-link = Geoffrey Colin Shephard
 | doi = 10.1112/blms/1.3.257
 | journal = [[Bulletin of the London Mathematical Society]]
 | mr = 0250188
 | pages = 257–300
 | title = Convex polytopes
 | url = http://www.wias-berlin.de/people/si/course/files/convex_polytopes-survey-Gruenbaum.pdf
 | volume = 1
 | issue = 3
 | year = 1969
 | access-date = 2017-02-21
 | archive-url = https://web.archive.org/web/20170222114014/http://www.wias-berlin.de/people/si/course/files/convex_polytopes-survey-Gruenbaum.pdf
 | archive-date = 2017-02-22
 }}. See in particular the bottom of page 260.</ref>

Abstract polyhedra also have duals, obtained by reversing the [[partial order]] defining the polyhedron to obtain its [[Duality (order theory)|dual or opposite order]].<ref name=grunbaum-same/> These have the same Euler characteristic and orientability as the initial polyhedron. However, this form of duality does not describe the shape of a dual polyhedron, but only its combinatorial structure. For some definitions of non-convex geometric polyhedra, there exist polyhedra whose abstract duals cannot be realized as geometric polyhedra under the same definition.<ref name=acoptic/>

===Vertex figures===
{{Main|Vertex figure}}
For every vertex one can define a [[vertex figure]], which describes the local structure of the polyhedron around the vertex. Precise definitions vary, but a vertex figure can be thought of as the polygon exposed where a slice through the polyhedron cuts off a vertex.<ref name=cromwell/> For the [[Platonic solid]]s and other highly-symmetric polyhedra, this slice may be chosen to pass through the midpoints of each edge incident to the vertex,<ref>{{citation| first = H. S. M. | last = Coxeter | author-link = Harold Scott MacDonald Coxeter|title=Regular Polytopes|title-link=Regular Polytopes (book)|publisher=Methuen|year=1947|page=[https://books.google.com/books?id=iWvXsVInpgMC&pg=PA16 16]}}</ref> but other polyhedra may not have a plane through these points. For convex polyhedra, and more generally for polyhedra whose vertices are in [[convex position]], this slice can be chosen as any plane separating the vertex from the other vertices.<ref>{{citation
 | last = Barnette | first = David
 | journal = Pacific Journal of Mathematics
 | mr = 328773
 | pages = 349–354
 | title = A proof of the lower bound conjecture for convex polytopes
 | url = https://projecteuclid.org/euclid.pjm/1102946311
 | volume = 46
 | year = 1973| issue = 2
 | doi = 10.2140/pjm.1973.46.349
 | doi-access = free
 }}</ref> When the polyhedron has a center of symmetry, it is standard to choose this plane to be perpendicular to the line through the given vertex and the center;<ref>{{citation
 | last = Luotoniemi | first = Taneli
 | editor1-last = Swart | editor1-first = David
 | editor2-last = Séquin | editor2-first = Carlo H.
 | editor3-last = Fenyvesi | editor3-first = Kristóf
 | contribution = Crooked houses: Visualizing the polychora with hyperbolic patchwork
 | contribution-url = https://archive.bridgesmathart.org/2017/bridges2017-17.html
 | isbn = 978-1-938664-22-9
 | location = Phoenix, Arizona
 | pages = 17–24
 | publisher = Tessellations Publishing
 | title = Proceedings of Bridges 2017: Mathematics, Art, Music, Architecture, Education, Culture
 | year = 2017}}</ref> with this choice, the shape of the vertex figure is determined up to scaling. When the vertices of a polyhedron are not in convex position, there will not always be a plane separating each vertex from the rest. In this case, it is common instead to slice the polyhedron by a small sphere centered at the vertex.<ref>{{citation
 | date = January 1930
 | doi = 10.1098/rsta.1930.0009
 | first = H. S. M. | last = Coxeter | author-link = Harold Scott MacDonald Coxeter
 | issue = 670–680
 | journal = Philosophical Transactions of the Royal Society of London, Series A
 | pages = 329–425
 | publisher = The Royal Society
 | title = The polytopes with regular-prismatic vertex figures
 | volume = 229| bibcode = 1930RSPTA.229..329C
 }}</ref> Again, this produces a shape for the vertex figure that is invariant up to scaling. All of these choices lead to vertex figures with the same combinatorial structure, for the polyhedra to which they can be applied, but they may give them different geometric shapes.

===Surface area and distances===
The [[surface area]] of a polyhedron is the sum of areas of its faces, for definitions of polyhedra for which the area of a face is well-defined.
The [[geodesic]] distance between any two points on the surface of a polyhedron measures the length of the shortest curve that connects the two points, remaining within the surface. By [[Alexandrov's uniqueness theorem]], every convex polyhedron is uniquely determined by the [[metric space]] of geodesic distances on its surface. However, non-convex polyhedra can have the same surface distances as each other, or the same as certain convex polyhedra.<ref>{{citation
 | last = Hartshorne | first = Robin | author-link = Robin Hartshorne
 | contribution = Example 44.2.3, the "punched-in icosahedron"
 | doi = 10.1007/978-0-387-22676-7
 | isbn = 0-387-98650-2
 | mr = 1761093
 | page = 442
 | publisher = Springer-Verlag, New York
 | series = Undergraduate Texts in Mathematics
 | title = Geometry: Euclid and beyond
 | year = 2000}}</ref>

===Volume===
Polyhedral solids have an associated quantity called [[volume]] that measures how much space they occupy.  Simple families of solids may have simple formulas for their volumes; for example, the volumes of pyramids, prisms, and [[parallelepiped]]s can easily be expressed in terms of their edge lengths or other coordinates.  (See [[volume#Formulas|Volume § Volume formulas]] for a list that includes many of these formulas.)

Volumes of more complicated polyhedra may not have simple formulas.  Volumes of such polyhedra may be computed by subdividing the polyhedron into smaller pieces (for example, by [[point-set triangulation|triangulation]]).  For example, the [[Platonic solid#Radii, area, and volume|volume of a regular polyhedron]] can be computed by dividing it into congruent [[Pyramid (geometry)|pyramids]], with each pyramid having a face of the polyhedron as its base and the centre of the polyhedron as its apex.

In general, it can be derived from the [[divergence theorem]] that the volume of a polyhedral solid is given by
<math display="block">
\frac{1}{3} \left| \sum_F (Q_F \cdot N_F) \operatorname{area}(F) \right|,
</math>
where the sum is over faces {{mvar|F}} of the polyhedron, {{math|''Q''<sub>''F''</sub>}} is an arbitrary point on face {{mvar|F}}, {{math|''N''<sub>''F''</sub>}} is the [[unit vector]] perpendicular to {{mvar|F}} pointing outside the solid, and the multiplication dot is the [[dot product]].<ref>{{citation |last=Goldman |first=Ronald N.|author-link=Ron Goldman (mathematician) |editor-last=Arvo |editor-first=James |title=Graphic Gems Package: Graphics Gems II |publisher=Academic Press |year=1991 |pages=170–171 |chapter=Chapter IV.1: Area of planar polygons and volume of polyhedra}}</ref> In higher dimensions, volume computation may be challenging, in part because of the difficulty of listing the faces of a convex polyhedron specified only by its vertices, and there exist specialized [[algorithm]]s to determine the volume in these cases.<ref>{{citation | last1 = Büeler | first1 = B. | last2 = Enge | first2 = A. | last3 = Fukuda | first3 = K. | doi = 10.1007/978-3-0348-8438-9_6 | chapter = Exact Volume Computation for Polytopes: A Practical Study | title = Polytopes — Combinatorics and Computation | pages = 131–154 | year = 2000 | isbn = 978-3-7643-6351-2 | citeseerx = 10.1.1.39.7700 }}</ref>

===Dehn invariant===
{{main|Dehn invariant}}
In two dimensions, the [[Bolyai–Gerwien theorem]] asserts that any polygon may be transformed into any other polygon of the same area by [[Dissection problem|cutting it up into finitely many polygonal pieces and rearranging them]].  The analogous question for polyhedra was the subject of [[Hilbert's third problem]]. [[Max Dehn]] solved this problem by showing that, unlike in the 2-D case, there exist polyhedra of the same volume that cannot be cut into smaller polyhedra and reassembled into each other. To prove this Dehn discovered another value associated with a polyhedron, the [[Dehn invariant]], such that two polyhedra can only be dissected into each other when they have the same volume and the same Dehn invariant. It was later proven by Sydler that this is the only obstacle to dissection: every two Euclidean polyhedra with the same volumes and Dehn invariants can be cut up and reassembled into each other.<ref>{{citation |last=Sydler |first=J.-P. | author-link = Jean-Pierre Sydler |title=Conditions nécessaires et suffisantes pour l'équivalence des polyèdres de l'espace euclidien à trois dimensions |journal=[[Commentarii Mathematici Helvetici|Comment. Math. Helv.]]|language=fr |volume=40 |year=1965 |pages=43–80 |doi= 10.1007/bf02564364| mr = 0192407|s2cid=123317371 | url = https://eudml.org/doc/139296 }}</ref> The Dehn invariant is not a number, but a [[Vector (mathematics)|vector]] in an infinite-dimensional vector space, determined from the lengths and [[dihedral angle]]s of a polyhedron's edges.<ref>{{SpringerEOM|first=M.|last=Hazewinkel|title=Dehn invariant|id=Dehn_invariant&oldid=35803}}</ref>

Another of Hilbert's problems, [[Hilbert's 18th problem]], concerns (among other things) polyhedra that [[Honeycomb (geometry)|tile space]]. Every such polyhedron must have Dehn invariant zero.<ref>{{citation
 | last = Debrunner | first = Hans E.
 | doi = 10.1007/BF01235384
 | issue = 6
 | journal = [[Archiv der Mathematik]]
 | language = de
 | mr = 604258
 | pages = 583–587
 | title = Über Zerlegungsgleichheit von Pflasterpolyedern mit Würfeln
 | volume = 35
 | year = 1980| s2cid = 121301319
 }}.</ref> The Dehn invariant has also been connected to [[flexible polyhedron|flexible polyhedra]] by the strong bellows theorem, which states that the Dehn invariant of any flexible polyhedron remains invariant as it flexes.<ref>{{citation
 | last = Alexandrov | first = Victor
 | arxiv = 0901.2989
 | doi = 10.1007/s00022-011-0061-7
 | issue = 1–2
 | journal = Journal of Geometry
 | mr = 2823098
 | pages = 1–13
 | title = The Dehn invariants of the Bricard octahedra
 | volume = 99
 | year = 2010| citeseerx = 10.1.1.243.7674
 | s2cid = 17515249
 }}.</ref>

==Symmetries==
[[File:Revolução de poliedros 03.webm|thumb|280px|Some polyhedra rotating around a symmetrical axis (at [[:pt:Matemateca IME-USP|Matemateca IME-USP]])]]
Many of the most studied polyhedra are highly [[Symmetry|symmetrical]], that is, their appearance is unchanged by some reflection or rotation of space.  Each such symmetry may change the location of a given vertex, face, or edge, but the set of all vertices (likewise faces, edges) is unchanged.  The collection of symmetries of a polyhedron is called its [[symmetry group]].

All the elements that can be superimposed on each other by symmetries are said to form a [[Symmetry orbit#Orbits and stabilizers|symmetry orbit]]. For example, all the faces of a cube lie in one orbit, while all the edges lie in another. If all the elements of a given dimension, say all the faces, lie in the same orbit, the figure is said to be transitive on that orbit. For example, a cube is face-transitive, while a [[truncated cube]] has two symmetry orbits of faces.

The same abstract structure may support more or less symmetric geometric polyhedra. But where a polyhedral name is given, such as [[icosidodecahedron]], the most symmetrical geometry is often implied.{{citation needed|date=February 2017}}

There are several types of highly symmetric polyhedron, classified by which kind of element – faces, edges, or vertices – belong to a single symmetry orbit:
* [[Regular polyhedron|Regular]]: vertex-transitive, edge-transitive and face-transitive.  (This implies that every face is the same [[regular polygon]]; it also implies that every vertex is regular.)
* [[Quasiregular polyhedron|Quasi-regular]]: vertex-transitive and edge-transitive (and hence has regular faces) but not face-transitive. A quasi-regular dual is face-transitive and edge-transitive (and hence every vertex is regular) but not vertex-transitive.
* [[Semiregular polyhedron|Semi-regular]]: vertex-transitive but not edge-transitive, and every face is a regular polygon. (This is one of several definitions of the term, depending on author. Some definitions overlap with the quasi-regular class.) These polyhedra include the semiregular [[prism (geometry)|prisms]] and [[antiprism]]s. A semi-regular dual is face-transitive but not vertex-transitive, and every vertex is regular.
* [[Uniform polyhedron|Uniform]]: vertex-transitive and every face is a regular polygon, i.e., it is regular, quasi-regular or semi-regular. A uniform dual is face-transitive and has regular vertices, but is not necessarily vertex-transitive.
* [[Isogonal figure|Isogonal]]: vertex-transitive.
* [[Isotoxal]]: edge-transitive.
* [[Isohedral]]: face-transitive.
* [[Noble polyhedron|Noble]]: face-transitive and vertex-transitive (but not necessarily edge-transitive). The regular polyhedra are also noble; they are the only noble uniform polyhedra.  The duals of noble polyhedra are themselves noble.

Some classes of polyhedra have only a single main axis of symmetry. These include the [[pyramid (geometry)|pyramids]], [[bipyramid]]s, [[trapezohedra]], [[cupola (geometry)|cupolae]], as well as the semiregular prisms and antiprisms.

===Regular polyhedra===
{{main|Regular polyhedron}}

Regular polyhedra are the most highly symmetrical. Altogether there are nine regular polyhedra: five convex and four star polyhedra.

The five convex examples have been known since antiquity and are called the [[Platonic solids]]. These are the triangular pyramid or [[tetrahedron]], [[cube]], [[octahedron]], [[dodecahedron]] and [[icosahedron]]:

{| class="wikitable" style="margin-left:1em"
|[[File:Tetrahedron.svg|50px]]
|[[File:Hexahedron.svg|50px]]
|[[File:Octahedron.svg|50px]]
|[[File:Dodecahedron.svg|50px]]
|[[File:Icosahedron.svg|50px]]
|}

There are also four regular star polyhedra, known as the [[Kepler–Poinsot polyhedra]] after their discoverers.

The dual of a regular polyhedron is also regular.

===Uniform polyhedra and their duals===
{{Main|Uniform polyhedron}}

Uniform polyhedra are [[vertex-transitive]] and every face is a [[regular polygon]].
They may be subdivided into the [[Regular polyhedron|regular]], [[Quasiregular polyhedron|quasi-regular]], or [[Semiregular polyhedron|semi-regular]], and may be convex or starry.

The duals of the uniform polyhedra have irregular faces but are [[face-transitive]], and every [[vertex figure]] is a regular polygon. A uniform polyhedron has the same symmetry orbits as its dual, with the faces and vertices simply swapped over. The duals of the convex Archimedean polyhedra are sometimes called the [[Catalan solid]]s.

The uniform polyhedra and their duals are traditionally classified according to their degree of symmetry, and whether they are [[Convex polyhedron|convex]] or not.

{| class="wikitable"
!
!Convex uniform
!Convex uniform dual
!Star uniform
!Star uniform dual
|-
![[Regular polyhedron|Regular]]
| style="text-align:center;" colspan="2"|[[Platonic solid]]s
| style="text-align:center;" colspan="2"|[[Kepler–Poinsot polyhedron|Kepler–Poinsot polyhedra]]
|- style="text-align:center;"
![[Quasiregular polyhedron|Quasiregular]]
|rowspan="2" |[[Archimedean solid]]s
|rowspan="2"|[[Catalan solid]]s
|colspan=2 rowspan=2|[[Uniform star polyhedron]]
|- style="text-align:center;"
!rowspan="3"|[[Semiregular polyhedron|Semiregular]]
|- style="text-align:center;"
|[[Prism (geometry)|Prisms]]
|[[Bipyramid]]s
|[[Star prism]]s
|[[Star bipyramid]]s
|- style="text-align:center;"
|[[Antiprism]]s
|[[Trapezohedron|Trapezohedra]]
|[[Star antiprism]]s
|[[Star trapezohedra]]
|}

===Isohedra===
{{main|Isohedron}}

An '''isohedron''' is a polyhedron with symmetries acting transitively on its faces. Their topology can be represented by a [[face configuration]]. All 5 [[Platonic solids]] and 13 [[Catalan solid]]s are isohedra, as well as the infinite families of [[trapezohedra]] and [[bipyramid]]s. Some definitions of isohedra allow geometric variations including concave and self-intersecting forms.

===Symmetry groups===
[[File:Icosahedral reflection domains.png|thumb|Full [[icosahedral symmetry]] divides the sphere into 120 triangular domains.]]
Many of the symmetries or [[point groups in three dimensions]] are named after polyhedra having the associated symmetry. These include:
* '''T''' – '''chiral [[tetrahedral symmetry]]'''; the rotation group for a regular [[tetrahedron]]; order 12.
* '''T<sub>d</sub>''' – '''full [[tetrahedral symmetry]]'''; the symmetry group for a regular [[tetrahedron]]; order 24.
* '''T<sub>h</sub>''' – '''[[tetrahedral symmetry|pyritohedral symmetry]]'''; the symmetry of a [[pyritohedron]]; order 24.
* '''O''' – '''chiral [[octahedral symmetry]]''';the rotation group of the [[Cube (geometry)|cube]] and [[octahedron]]; order 24.
* '''O<sub>h</sub>''' – '''full [[octahedral symmetry]]'''; the symmetry group of the [[Cube (geometry)|cube]] and [[octahedron]]; order 48.
* '''I''' – '''chiral [[icosahedral symmetry]]'''; the rotation group of the [[icosahedron]] and the [[dodecahedron]]; order 60.
* '''I<sub>h</sub>''' – '''full [[icosahedral symmetry]]'''; the symmetry group of the [[icosahedron]] and the [[dodecahedron]]; order 120.
* '''C<sub>nv</sub>''' – [[Cyclic symmetries|''n''-fold pyramidal symmetry]]
* '''D<sub>nh</sub>''' – [[Dihedral symmetry|''n''-fold prismatic symmetry]]
* '''D<sub>nv</sub>''' – [[Dihedral symmetry|''n''-fold antiprismatic symmetry]].

Those with [[Chirality (mathematics)|chiral]] symmetry do not have [[reflection symmetry]] and hence have two enantiomorphous forms which are reflections of each other. Examples include the [[snub cuboctahedron]] and [[snub icosidodecahedron]].

== Other important families of polyhedra ==
=== Zonohedra ===
{{main|Zonohedron}}
A zonohedron is a convex polyhedron in which every face is a [[polygon]] that is symmetric under [[rotation]]s through 180°. Zonohedra can also be characterized as the [[Minkowski sum]]s of line segments, and include several important space-filling polyhedra.<ref>{{citation
 | last = Taylor | first = Jean E. | author-link = Jean Taylor
 | doi = 10.2307/2324178
 | issue = 2
 | journal = [[American Mathematical Monthly]]
 | mr = 1144350
 | pages = 108–111
 | title = Zonohedra and generalized zonohedra
 | volume = 99
 | year = 1992| jstor = 2324178 }}.</ref>

===Space-filling polyhedra===
{{Main|Honeycomb (geometry)}}
A space-filling polyhedron packs with copies of itself to fill space. Such a close-packing or space-filling is often called a tessellation of space or a honeycomb. Space-filling polyhedra must have a [[#Dehn invariant|Dehn invariant]] equal to zero. Some honeycombs involve more than one kind of polyhedron.

=== Lattice polyhedra ===
{{main|Ehrhart polynomial}}
A convex polyhedron in which all vertices have integer coordinates is called a [[convex lattice polytope|lattice polyhedron]] or [[integral polyhedron]].  The Ehrhart polynomial of a lattice polyhedron counts how many points with [[integer]] coordinates lie within a scaled copy of the polyhedron, as a function of the scale factor. The study of these polynomials lies at the intersection of [[combinatorics]] and [[commutative algebra]].<ref>{{citation|last=Stanley |first=Richard P. |date=1997 |title=Enumerative Combinatorics, Volume I |edition=1 |publisher=Cambridge University Press |pages=235–239 |isbn=978-0-521-66351-9 |author-link=Richard P. Stanley }}</ref> There is a far-reaching equivalence between lattice polyhedra and certain [[Algebraic variety|algebraic varieties]] called [[Toric variety|toric varieties]].<ref>{{citation |last=Cox |first=David A. |title=Toric varieties |date=2011 |publisher=American Mathematical Society |others=John B. Little, Henry K. Schenck |isbn=978-0-8218-4819-7 |location=Providence, R.I. |oclc=698027255}}</ref> This was used by Stanley to prove the [[Dehn–Sommerville equations]] for [[Simplicial polytope|simplicial polytopes]].<ref>{{citation |last=Stanley |first=Richard P. |title=Combinatorics and commutative algebra |date=1996 |publisher=Birkhäuser |isbn=0-8176-3836-9 |edition=2nd |location=Boston |oclc=33080168}}</ref>

=== Flexible polyhedra ===
{{main|Flexible polyhedron}}
It is possible for some polyhedra to change their overall shape, while keeping the shapes of their faces the same, by varying the angles of their edges. A polyhedron that can do this is called a flexible polyhedron. By [[Cauchy's theorem (geometry)|Cauchy's rigidity theorem]], flexible polyhedra must be non-convex. The volume of a flexible polyhedron must remain constant as it flexes; this result is known as the bellows theorem.<ref>{{citation
 | last1 = Demaine | first1 = Erik D. | author1-link = Erik Demaine
 | last2 = O'Rourke | first2 = Joseph | author2-link = Joseph O'Rourke (professor)
 | contribution = 23.2 Flexible polyhedra
 | doi = 10.1017/CBO9780511735172
 | isbn = 978-0-521-85757-4
 | mr = 2354878
 | pages = 345–348
 | publisher = Cambridge University Press, Cambridge
 | title = Geometric Folding Algorithms: Linkages, origami, polyhedra
 | title-link=Geometric Folding Algorithms
 | year = 2007}}.</ref>

=== Compounds ===
{{main|Polyhedral compound}}
A polyhedral compound is made of two or more polyhedra sharing a common centre.  Symmetrical compounds often share the same vertices as other well-known polyhedra and may often also be formed by stellation. Some are listed in the [[list of Wenninger polyhedron models]].

=== Orthogonal polyhedra ===
[[File:Soma_cube_figures.svg|thumb|Some orthogonal polyhedra made of [[Soma cube]] pieces, themselves [[polycube]]s]]
An orthogonal polyhedron is one all of whose edges are parallel to axes of a Cartesian coordinate system. This implies that all faces meet at [[right angle]]s, but this condition is weaker: [[Jessen's icosahedron]] has faces meeting at right angles, but does not have axis-parallel edges.

Aside from the [[rectangular cuboid]]s, orthogonal polyhedra are nonconvex. They are the 3D analogs of 2D orthogonal polygons, also known as [[rectilinear polygon]]s.  Orthogonal polyhedra are used in [[computational geometry]], where their constrained structure has enabled advances on problems unsolved for arbitrary polyhedra, for example, unfolding the surface of a polyhedron to a [[polygonal net]].<ref>{{citation
 | last = O'Rourke | first = Joseph | author-link = Joseph O'Rourke (professor)
 | contribution = Unfolding orthogonal polyhedra
 | doi = 10.1090/conm/453/08805
 | mr = 2405687
 | pages = 307–317
 | publisher = Amer. Math. Soc., Providence, RI
 | series = Contemp. Math.
 | title = Surveys on discrete and computational geometry
 | volume = 453
 | year = 2008| isbn =978-0-8218-4239-3 | doi-access = free
 }}.</ref>

[[Polycube]]s are a special case of orthogonal polyhedra that can be decomposed into identical cubes, and are three-dimensional analogues of planar [[polyomino]]es.<ref>{{citation
 | last = Gardner | first = Martin | author-link = Martin Gardner
 | date = November 1966
 | issue = 5
 | journal = [[Scientific American]]
 | jstor = 24931332
 | pages = 138–143
 | title = Mathematical Games: Is it possible to visualize a four-dimensional figure?
 | volume = 215| doi = 10.1038/scientificamerican1166-138 }}</ref>

=== Embedded regular maps with planar faces ===
{{Multiple image|total_width = 400
|image1=Heawood map on a hexagon.svg|caption1=The [[Heawood graph|Heawood map]], a regular map on a topological torus formed by gluing opposite edges of the outer hexagon|image2=Szilassi polyhedron.svg
|caption2=The Szilassi polyhedron, a polyhedron realizing the Heawood map}}
{{main|Regular map (graph theory)}}
[[Regular map (graph theory)|Regular map]]s are flag transitive abstract 2-manifolds and they have been studied already in the nineteenth century. In some cases they have geometric realizations. An example is the [[Szilassi polyhedron]], a toroidal polyhedron that realizes the [[Heawood graph|Heawood map]]. In this case, the polyhedron is much less symmetric than the underlying map, but in some cases it is possible for self-crossing polyhedra to realize some or all of the symmetries of a regular map.
{{-}}

==Generalisations==
The name 'polyhedron' has come to be used for a variety of objects having similar structural properties to traditional polyhedra.

===Apeirohedra===
A classical polyhedral surface has a finite number of faces, joined in pairs along edges. The [[apeirohedron|apeirohedra]] form a related class of objects with infinitely many faces. Examples of apeirohedra include:
* tilings or [[tessellation]]s of the plane, and
* sponge-like structures called [[skew apeirohedron|infinite skew polyhedra]].

===Complex polyhedra===
{{Main|Complex polytope}}
There are objects called complex polyhedra, for which the underlying space is a [[complex number|complex]] [[Hilbert space]] rather than real Euclidean space.  Precise definitions exist only for the regular complex polyhedra, whose symmetry groups are [[complex reflection group]]s.  The complex polyhedra are mathematically more closely related to [[configuration (polytope)|configurations]] than to real polyhedra.<ref>{{citation
 | last = Coxeter | first = H.S.M. | author-link = Harold Scott MacDonald Coxeter
 | location = Cambridge
 | mr = 0370328
 | publisher = Cambridge University Press
 | title = Regular Complex Polytopes
 | year = 1974}}. {{page needed|date=February 2017}}</ref>

===Curved polyhedra===
Some fields of study allow polyhedra to have curved faces and edges. Curved faces can allow [[digon]]al faces to exist with a positive area.

====Spherical polyhedra====
{{Main|Spherical polyhedron}}

When the surface of a sphere is divided by finitely many [[great arc]]s (equivalently, by planes passing through the center of the sphere), the result is called a spherical polyhedron.  Many convex polytopes having some degree of symmetry (for example, all the Platonic solids) can be projected onto the surface of a concentric sphere to produce a spherical polyhedron.  However, the reverse process is not always possible; some spherical polyhedra (such as the [[hosohedron|hosohedra]]) have no flat-faced analogue.<ref>{{citation|title=Divided Spheres: Geodesics and the Orderly Subdivision of the Sphere|first=Edward S.|last=Popko|publisher=CRC Press|year=2012|isbn=978-1-4665-0430-1|page=463|url=https://books.google.com/books?id=HjTSBQAAQBAJ&pg=PA463|quote="A hosohedron is only possible on a sphere"}}.</ref>

====Curved spacefilling polyhedra====
If faces are allowed to be concave as well as convex, adjacent faces may be made to meet together with no gap. Some of these curved polyhedra can pack together to fill space. Two important types are:
* Bubbles in froths and foams, such as [[Weaire–Phelan structure|Weaire-Phelan bubbles]].<ref>{{citation
 | last1 = Kraynik | first1 = A.M.
 | last2 = Reinelt | first2 = D.A.
 | editor-last = Mortensen | editor-first = Andreas
 | contribution = Foams, Microrheology of
 | edition = 2nd
 | pages = 402–407
 | publisher = Elsevier
 | title = Concise Encyclopedia of Composite Materials
 | year = 2007}}. See in particular [https://books.google.com/books?id=zs_lGeGsuaAC&pg=PA403 p.&nbsp;403]: "foams consist of polyhedral gas bubbles ... each face on a polyhedron is a minimal surface with uniform mean curvature ... no face can be a flat polygon with straight edges".</ref>
* Forms used in architecture.<ref>{{citation|last=Pearce|first=P.|title=Structure in nature is a strategy for design|publisher=MIT Press|year=1978|page=224|url=https://books.google.com/books?id=sfc2OEuE8oQC&pg=PA224|contribution=14 Saddle polyhedra and continuous surfaces as environmental structures|isbn=978-0-262-66045-7}}.</ref>
<!--More needs to be said about these, too.-->

===Ideal polyhedra===
{{main|Ideal polyhedron}}
Convex polyhedra can be defined in three-dimensional [[hyperbolic space]] in the same way as in Euclidean space, as the [[convex hull]]s of finite sets of points.
However, in hyperbolic space, it is also possible to consider [[ideal point]]s as well as the points that lie within the space. An [[ideal polyhedron]] is the convex hull of a finite set of ideal points. Its faces are ideal polygons, but its edges are defined by entire hyperbolic lines rather than line segments, and its vertices (the ideal points of which it is the convex hull) do not lie within the hyperbolic space.

===Skeletons and polyhedra as graphs===
By forgetting the face structure, any polyhedron gives rise to a [[Graph (discrete mathematics)|graph]], called its [[n-skeleton|skeleton]], with corresponding vertices and edges.  Such figures have a long history: [[Leonardo da Vinci]] devised frame models of the regular solids, which he drew for [[Pacioli]]'s book ''Divina Proportione'', and similar [[Wire-frame model|wire-frame]] polyhedra appear in [[M.C. Escher]]'s print [[Stars (M. C. Escher)|''Stars'']].<ref>{{citation | last=Coxeter | first=H.S.M. | author-link=Harold Scott MacDonald Coxeter | doi=10.1007/BF03023010 | issue=1 | journal=The Mathematical Intelligencer | pages=59–69 | title=A special book review: M.C. Escher: His life and complete graphic work | volume=7 | year=1985| s2cid=189887063 }} Coxeter's analysis of ''Stars'' is on pp. 61–62.</ref>  One highlight of this approach is [[Steinitz's theorem]], which gives a purely graph-theoretic characterization of the skeletons of convex polyhedra: it states that the skeleton of every convex polyhedron is a [[vertex connectivity|3-connected]] [[planar graph]], and every 3-connected planar graph is the skeleton of some convex polyhedron.

An early idea of [[#Abstract polyhedra|abstract polyhedra]] was developed in [[Branko Grünbaum]]'s study of "hollow-faced polyhedra."  Grünbaum defined faces to be cyclically ordered sets of vertices, and allowed them to be [[Skew polygon|skew]] as well as planar.<ref name=sin/>

The graph perspective allows one to apply [[Glossary of graph theory|graph terminology]] and properties to polyhedra.  For example, the tetrahedron and [[Császár polyhedron]] are the only known polyhedra whose skeletons are [[complete graph]]s (K<sub>4</sub>), and various symmetry restrictions on polyhedra give rise to skeletons that are [[symmetric graph]]s.

==Alternative usages==
From the latter half of the twentieth century, various mathematical constructs have been found to have properties also present in traditional polyhedra. Rather than confining the term "polyhedron" to describe a three-dimensional polytope, it has been adopted to describe various related but distinct kinds of structure.

=== Higher-dimensional polyhedra ===
{{Main|n-dimensional polyhedron}}
A polyhedron has been defined as a set of points in [[real number|real]] [[affine space|affine]] (or [[Euclidean space|Euclidean]]) space of any dimension ''n'' that has flat sides.  It may alternatively be defined as the intersection of finitely many [[half-space (geometry)|half-spaces]].  Unlike a conventional polyhedron, it may be bounded or unbounded. In this meaning, a [[polytope]] is a bounded polyhedron.<ref name="polytope-bounded-1" /><ref name="polytope-bounded-2" />

Analytically, such a convex polyhedron is expressed as the solution set for a system of linear inequalities. Defining polyhedra in this way provides a geometric perspective for problems in [[linear programming]].<ref name=":0">{{Cite Geometric Algorithms and Combinatorial Optimization}}</ref>{{Rp|page=9}} 
=== Topological polyhedra ===
A topological polytope is a topological space given along with a specific decomposition into shapes that are topologically equivalent to [[convex polytope]]s and that are attached to each other in a regular way.

Such a figure is called ''simplicial'' if each of its regions is a [[simplex]], i.e. in an ''n''-dimensional space each region has ''n''+1 vertices.  The dual of a simplicial polytope is called ''simple''.  Similarly, a widely studied class of polytopes (polyhedra) is that of cubical polyhedra, when the basic building block is an ''n''-dimensional cube.

===Abstract polyhedra===
{{Main|Abstract polytope}}

An [[abstract polytope]] is a [[partially ordered set]] (poset) of elements whose partial ordering obeys certain rules of incidence (connectivity) and ranking. The elements of the set correspond to the vertices, edges, faces and so on of the polytope: vertices have rank 0, edges rank 1, etc. with the partially ordered ranking corresponding to the dimensionality of the geometric elements. The empty set, required by set theory, has a rank of −1 and is sometimes said to correspond to the null polytope. An abstract polyhedron is an abstract polytope having the following ranking:
* rank 3: The [[polyhedron|maximal element]], sometimes identified with the body.
* rank 2: The [[face (geometry)|polygonal faces]].
* rank 1: The [[edge (geometry)|edges]].
* rank 0: the [[vertex (geometry)|vertices]].
* rank −1: The empty set, sometimes identified with the {{vanchor|null polytope|null_polytope|nullitope|text=''null polytope'' or ''nullitope''}}.<ref>[[Norman Johnson (mathematician)|N.W. Johnson]]: ''Geometries and Transformations'', (2018) {{ISBN|978-1-107-10340-5}} Chapter 11: ''Finite Symmetry Groups'', 11.1 ''Polytopes and Honeycombs'', p.224</ref>

Any geometric polyhedron is then said to be a "realization" in real space of the abstract poset as described above.<!-- but see discussion -->

==History==
===Before the Greeks===
[[File:Papyrus moscow 4676-problem 14 part 1.jpg|thumb|Problem 14 of the [[Moscow Mathematical Papyrus]], on calculating the volume of a [[frustum]]]]
Polyhedra appeared in early [[architecture|architectural forms]] such as cubes and cuboids, with the earliest four-sided [[Egyptian pyramids]] dating from the [[27th century BC]].<ref>{{citation | last = Kitchen | first = K. A. | date = October 1991 | doi = 10.1080/00438243.1991.9980172 | issue = 2 | journal = World Archaeology | pages = 201–208 | title = The chronology of ancient Egypt | volume = 23}}</ref> The [[Moscow Mathematical Papyrus]] from approximately 1800–1650 BC includes an early written study of polyhedra and their volumes (specifically, the volume of a [[frustum]]).<ref>{{citation | last1 = Gunn | first1 = Battiscombe | last2 = Peet | first2 = T. Eric | date = May 1929 | doi = 10.1177/030751332901500130 | issue = 1 | journal = The Journal of Egyptian Archaeology | pages = 167–185 | title = Four Geometrical Problems from the Moscow Mathematical Papyrus | volume = 15| s2cid = 192278129 }}</ref> The mathematics of the [[Old Babylonian Empire]], from roughly the same time period as the Moscow Papyrus, also included calculations of the volumes of [[cuboid]]s (and of non-polyhedral [[cylinder]]s), and calculations of the height of such a shape needed to attain a given volume.<ref>{{citation | last = Friberg | first = Jöran | issue = 2 | journal = Revue d'Assyriologie et d'archéologie orientale | jstor = 23281940 | pages = 97–188 | title = Mathematics at Ur in the Old Babylonian Period | volume = 94 | year = 2000}}</ref>

The [[Etruscan civilization|Etruscans]] preceded the Greeks in their awareness of at least some of the regular polyhedra, as evidenced by the discovery of an [[Etruscan civilization|Etruscan]]  [[dodecahedron]] made of [[soapstone]] on [[Monte Loffa]]. Its faces were marked with different designs, suggesting to some scholars that it may have been used as a gaming die.<ref>{{citation |title=An Etruscan dodecahedron|first=Amelia Carolina|last=Sparavigna|year=2012|arxiv=1205.0706}}</ref>

===Ancient Greece===
Ancient Greek mathematicians discovered and studied the [[Regular polyhedron#History|convex regular polyhedra]], which came to be known as the [[Platonic solid]]s. Their first written description is in the ''[[Timaeus (dialogue)|Timaeus]]'' of [[Plato]] (circa 360 BC), which associates four of them with the [[four elements]] and the fifth to the overall shape of the universe. A more mathematical treatment of these five polyhedra was written soon after in the ''[[Euclid's Elements|Elements]]'' of [[Euclid]]. An early commentator on Euclid (possibly [[Geminus]]) writes that the attribution of these shapes to Plato is incorrect: [[Pythagoras]] knew the [[tetrahedron]], [[cube]], and [[dodecahedron]], and [[Theaetetus (mathematician)|Theaetetus]] (circa 417 BC) discovered the other two, the [[octahedron]] and [[icosahedron]].<ref>{{citation | last = Eves | first = Howard | date = January 1969 | department = Historically Speaking | doi = 10.5951/mt.62.1.0042 | issue = 1 | journal = The Mathematics Teacher | jstor = 27958041 | pages = 42–44 | title = A geometry capsule concerning the five platonic solids | volume = 62}}</ref> Later, [[Archimedes]] expanded his study to the [[Uniform polyhedron|convex uniform polyhedra]] which now bear his name. His original work is lost and his solids come down to us through [[Pappus of Alexandria|Pappus]].<ref>{{citation | last = Field | first = J. V. | author-link = Judith V. Field | doi = 10.1007/BF00374595 | issue = 3–4 | journal = Archive for History of Exact Sciences | jstor = 41134110 | mr = 1457069 | pages = 241–289 | title = Rediscovering the Archimedean polyhedra: Piero della Francesca, Luca Pacioli, Leonardo da Vinci, Albrecht Dürer, Daniele Barbaro, and Johannes Kepler | volume = 50 | year = 1997| s2cid = 118516740 }}</ref>

===Ancient China===
[[File:14-sided Chinese dice from warring states period.jpg|thumb|upright|14-sided die from the [[Warring States period]]]]
Both cubical dice and 14-sided dice in the shape of a [[truncated octahedron]] in China have been dated back as early as the [[Warring States period]].<ref>{{citation
 | last1 = Bréard | first1 = Andrea | author1-link = Andrea Bréard
 | last2 = Cook | first2 = Constance A.
 | date = December 2019
 | doi = 10.1007/s00407-019-00245-9
 | issue = 4
 | journal = Archive for History of Exact Sciences
 | pages = 313–343
 | title = Cracking bones and numbers: solving the enigma of numerical sequences on ancient Chinese artifacts
 | volume = 74| s2cid = 253898304 }}</ref>

By 236 AD, [[Liu Hui]] was describing the dissection of the cube into its characteristic tetrahedron ([[orthoscheme]]) and related solids, using assemblages of these solids as the basis for calculating volumes of earth to be moved during engineering excavations.<ref>{{citation
 | last = van der Waerden | first = B. L.
 | contribution = Chapter 7: Liu Hui and Aryabhata
 | doi = 10.1007/978-3-642-61779-9_7
 | pages = 192–217
 | publisher = Springer
 | title = Geometry and Algebra in Ancient Civilizations
 | year = 1983}}</ref>

===Medieval Islam===
After the end of the Classical era, scholars in the Islamic civilisation continued to take the Greek knowledge forward (see [[Mathematics in medieval Islam]]).<ref>{{citation | last = Knorr | first = Wilbur | author-link = Wilbur Knorr | doi = 10.1016/0315-0860(83)90034-4 | issue = 1 | journal = Historia Mathematica | mr = 698139 | pages = 71–78 | title = On the transmission of geometry from Greek into Arabic | volume = 10 | year = 1983| doi-access = free }}</ref> The 9th century scholar [[Thabit ibn Qurra]] included the calculation of volumes in his studies,<ref>{{citation | last = Rashed | first = Roshdi | editor-last = Rashed | editor-first = Roshdi | contribution = Thābit ibn Qurra et l'art de la mesure | contribution-url = https://books.google.com/books?id=V5PTZi77YxwC&pg=PA173 | language = fr | isbn = 9783110220780 | pages = 173–175 | publisher = Walter de Gruyter | series = Scientia Graeco-Arabica | title = Thābit ibn Qurra: Science and Philosophy in Ninth-Century Baghdad | volume = 4 | year = 2009}}</ref> and wrote a work on the [[cuboctahedron]]. Then in the 10th century [[Abūl Wafā' Būzjānī|Abu'l Wafa]] described the convex regular and quasiregular spherical polyhedra.<ref>{{citation | last1 = Hisarligil | first1 = Hakan | last2 = Hisarligil | first2 = Beyhan Bolak | date = December 2017 | doi = 10.1007/s00004-017-0363-7 | issue = 1 | journal = Nexus Network Journal | pages = 125–152 | title = The geometry of cuboctahedra in medieval art in Anatolia | volume = 20| doi-access = free }}</ref>

===Renaissance===
{{multiple image
 | image1 = Pacioli.jpg
 | caption1 = ''[[Portrait of Luca Pacioli|Doppio ritratto]]'', attributed to [[Jacopo de' Barbari]], depicting [[Luca Pacioli]] and a student studying a glass [[rhombicuboctahedron]] half-filled with water.<ref>{{citation | last = Gamba | first = Enrico | title = Imagine Math | editor-last = Emmer | editor-first = Michele | contribution = The mathematical ideas of Luca Pacioli depicted by Iacopo de' Barbari in the ''Doppio ritratto'' | doi = 10.1007/978-88-470-2427-4_25 | isbn = 978-88-470-2427-4 | pages = 267–271 | publisher = Springer | year = 2012}}</ref>
 | image2 = Leonardo polyhedra.png
 | caption2 = A skeletal polyhedron (specifically, a [[rhombicuboctahedron]]) drawn by [[Leonardo da Vinci]] to illustrate a book by [[Luca Pacioli]]
 | total_width = 400
}}
As with other areas of Greek thought maintained and enhanced by Islamic scholars, Western interest in polyhedra revived during the Italian [[Renaissance]]. Artists constructed skeletal polyhedra, depicting them from life as a part of their investigations into [[Perspective (graphical)|perspective]].<ref name=polyhedrists>{{citation|title=The Polyhedrists: Art and Geometry in the Long Sixteenth Century|first=Noam|last=Andrews|publisher=MIT Press|year=2022|isbn=9780262046640}}</ref> [[Toroidal polyhedron|Toroidal polyhedra]], made of wood and used to support headgear, became a common exercise in perspective drawing, and were depicted in [[marquetry]] panels of the period as a symbol of geometry.<ref>{{citation | last1 = Calvo-López | first1 = José | last2 = Alonso-Rodríguez | first2 = Miguel Ángel | date = February 2010 | doi = 10.1007/s00004-010-0018-4 | issue = 1 | journal = Nexus Network Journal | pages = 75–111 | title = Perspective versus stereotomy: From Quattrocento polyhedral rings to sixteenth-century Spanish torus vaults | volume = 12| doi-access = free }}</ref> [[Piero della Francesca]] wrote about constructing perspective views of polyhedra, and rediscovered many of the Archimedean solids. [[Leonardo da Vinci]] illustrated skeletal models of several polyhedra for a book by [[Luca Pacioli]],<ref>{{citation | last = Field | first = J. V. | authorlink = Judith V. Field | doi = 10.1007/BF00374595 | issue = 3–4 | journal = [[Archive for History of Exact Sciences]] | jstor = 41134110 | mr = 1457069 | pages = 241–289 | s2cid = 118516740 | title = Rediscovering the Archimedean polyhedra: Piero della Francesca, Luca Pacioli, Leonardo da Vinci, Albrecht Dürer, Daniele Barbaro, and Johannes Kepler | volume = 50 | year = 1997}}</ref> with text largely plagiarized from della Francesca.<ref>{{citation | last = Montebelli | first = Vico | doi = 10.1007/s40329-015-0090-4 | issue = 3 | journal = Lettera Matematica | mr = 3402538 | pages = 135–141 | title = Luca Pacioli and perspective (part I) | volume = 3 | year = 2015 | s2cid = 193533200}}</ref> [[Polyhedral net]]s make an appearance in the work of [[Albrecht Dürer]].<ref>{{citation | last = Ghomi | first = Mohammad | issue = 1 | journal = [[Notices of the American Mathematical Society]] | mr = 3726673 | pages = 25–27 | title = Dürer's unfolding problem for convex polyhedra | url = https://www.ams.org/publications/journals/notices/201801/rnoti-p25.pdf | volume = 65 | year = 2018| doi = 10.1090/noti1609 }}</ref>

Several works from this time investigate star polyhedra, and other elaborations of the basic Platonic forms. A marble tarsia in the floor of [[St. Mark's Basilica]], Venice, designed by [[Paolo Uccello]], depicts a stellated dodecahedron.<ref>{{citation | last = Saffaro | first = Lucio | editor1-last = Taliani | editor1-first = C. | editor2-last = Ruani | editor2-first = G. | editor3-last = Zamboni | editor3-first = R. | contribution = Cosmoids, fullerenes and continuous polygons | contribution-url = https://books.google.com/books?id=dOk7DwAAQBAJ&pg=PA55 | location = Singapore | pages = 55–64 | publisher = World Scientific | title = Fullerenes: Status and Perspectives, Proceedings of the 1st Italian Workshop, Bologna, Italy, 6–7 February | year = 1992}}</ref> As the Renaissance spread beyond Italy, later artists such as [[Wenzel Jamnitzer]], Dürer and others also depicted polyhedra of increasing complexity, many of them novel, in imaginative etchings.<ref name=polyhedrists/> [[Johannes Kepler]] (1571–1630) used [[star polygon]]s, typically [[pentagram]]s, to build star polyhedra. Some of these figures may have been discovered before Kepler's time, but he was the first to recognize that they could be considered "regular" if one removed the restriction that regular polyhedra must be convex.<ref>{{citation | last = Field | first = J. V. | author-link = Judith V. Field | doi = 10.1016/0083-6656(79)90001-1 | issue = 2 | journal = Vistas in Astronomy | mr = 546797 | pages = 109–141 | title = Kepler's star polyhedra | volume = 23 | year = 1979| bibcode = 1979VA.....23..109F }}</ref>

In the same period, [[Euler's polyhedral formula]], a [[linear equation]] relating the numbers of vertices, edges, and faces of a polyhedron, was stated for the Platonic solids in 1537 in an unpublished manuscript by [[Francesco Maurolico]].<ref>{{citation|first= Michael|last=Friedman|publisher=Birkhäuser|year=2018|title=A History of Folding in Mathematics: Mathematizing the Margins|title-link=A History of Folding in Mathematics|series=Science Networks. Historical Studies|volume=59|isbn=978-3-319-72486-7|doi=10.1007/978-3-319-72487-4|page=71}}</ref>

===17th–19th centuries===
[[René Descartes]], in around 1630, wrote his book ''[[De solidorum elementis]]'' studying convex polyhedra as a general concept, not limited to the Platonic solids and their elaborations. The work was lost, and not rediscovered until the 19th century. One of its contributions was [[Descartes' theorem on total angular defect]], which is closely related to Euler's polyhedral formula.<ref>{{citation | last = Federico | first = Pasquale Joseph | isbn = 0-387-90760-2 | mr = 680214 | publisher = Springer-Verlag | series = Sources in the History of Mathematics and Physical Sciences | title = Descartes on Polyhedra: A Study of the "De solidorum elementis" | volume = 4 | year = 1982}}</ref> [[Leonhard Euler]], for whom the formula is named, introduced it in 1758 for convex polyhedra more generally, albeit with an incorrect proof.<ref>{{citation
 | last1 = Francese | first1 = Christopher
 | last2 = Richeson | first2 = David
 | doi = 10.1080/00029890.2007.11920417
 | issue = 4
 | journal = [[The American Mathematical Monthly]]
 | mr = 2281926
 | pages = 286–296
 | title = The flaw in Euler's proof of his polyhedral formula
 | volume = 114
 | year = 2007| s2cid = 10023787
 }}</ref> Euler's work (together with his earlier solution to the puzzle of the [[Seven Bridges of Königsberg]]) became the foundation of the new field of [[topology]].<ref>{{citation
 | last = Alexanderson | first = Gerald L.
 | doi = 10.1090/S0273-0979-06-01130-X
 | issue = 4
 | journal = American Mathematical Society
 | mr = 2247921
 | pages = 567–573
 | series = New Series
 | title = About the cover: Euler and Königsberg's bridges: a historical view
 | volume = 43
 | year = 2006| doi-access = free
 }}</ref> The core concepts of this field, including generalizations of the polyhedral formula, were developed in the late nineteenth century by [[Henri Poincaré]], [[Enrico Betti]], [[Bernhard Riemann]], and others.<ref>{{citation | last = Eckmann | first = Beno | contribution = The Euler characteristic – a few highlights in its long history | doi = 10.1007/978-3-540-33791-1_15 | isbn = 978-3-540-33791-1 | mr = 2269092 | pages = 177–188 | publisher = Springer | title = Mathematical Survey Lectures 1943–2004 | year = 2006}}</ref>

In the early 19th century, [[Louis Poinsot]] extended Kepler's work, and discovered the remaining two regular star polyhedra. Soon after, [[Augustin-Louis Cauchy]] proved Poinsot's list complete, subject to an unstated assumption that the sequence of vertices and edges of each polygonal side cannot admit repetitions (an assumption that had been considered but rejected in the earlier work of A. F. L. Meister).<ref>{{citation | last = Grünbaum | first = Branko | editor-last = Grattan-Guinness | editor-first = I. | contribution = Regular polyhedra | contribution-url = https://books.google.com/books?id=ZptYDwAAQBAJpg | isbn = 0-415-03785-9 | mr = 1469978 | pages = 866–876 | publisher = Routledge | title = Companion Encyclopedia of the History and Philosophy of the Mathematical Sciences | volume = 2 | year = 1994}}</ref> They became known as the [[Kepler–Poinsot polyhedra]], and their usual names were given by [[Arthur Cayley]].<ref>{{citation|title=Regular-faced polyhedra: remembering Norman Johnson|work=AMS Feature column|first=Joseph|last=Malkevitch|year=2018|url=https://www.ams.org/publicoutreach/feature-column/fc-2018-01|publisher=American Mathematical Society|access-date=2023-05-27}}
</ref> Meanwhile, the discovery of higher dimensions in the early 19th century led [[Ludwig Schläfli]] by 1853 to the idea of higher-dimensional polytopes.{{sfnp|Coxeter|1947|pages=141–143}} Additionally, in the late 19th century, Russian crystallographer [[Evgraf Fedorov]] completed the classification of [[Parallelohedron|parallelohedra]], convex polyhedra that tile space by translations.<ref>{{citation|last=Austin|first=David|title=Fedorov's five parallelohedra|work=AMS Feature Column|publisher=American Mathematical Society|url=https://www.ams.org/samplings/feature-column/fc-2013-11|date=November 2013}}</ref>

===20th–21st centuries===
Mathematics in the 20th century dawned with [[Hilbert's problems]], one of which, [[Hilbert's third problem]], concerned polyhedra and their [[Dissection problem|dissections]]. It was quickly solved by Hilbert's student [[Max Dehn]], introducing the [[Dehn invariant]] of polyhedra.<ref>{{citation
 | last = Zeeman | first = E. C. | author-link = Christopher Zeeman
 | date = July 2002
 | doi = 10.2307/3621846
 | issue = 506
 | journal = [[The Mathematical Gazette]]
 | jstor = 3621846
 | pages = 241–247
 | title = On Hilbert's third problem
 | volume = 86| s2cid = 125593771 }}</ref> [[Steinitz's theorem]], published by [[Ernst Steinitz]] in 1992, characterized the graphs of convex polyhedra, bringing modern ideas from [[graph theory]] and [[combinatorics]] into the study of polyhedra.<ref>{{citation
 | last = Grünbaum | first = Branko | author-link = Branko Grünbaum
 | doi = 10.1016/j.disc.2005.09.037
 | hdl = 1773/2276 | hdl-access = free
 | issue = 3–5
 | journal = [[Discrete Mathematics (journal)|Discrete Mathematics]]
 | mr = 2287486
 | pages = 445–463
 | title = Graphs of polyhedra; polyhedra as graphs
 | volume = 307
 | year = 2007}}</ref>

The Kepler–Poinsot polyhedra may be constructed from the Platonic solids by a process called [[stellation]]. Most stellations are not regular. The study of stellations of the Platonic solids was given a big push by [[H.S.M. Coxeter]] and others in 1938, with the now famous paper ''The 59 icosahedra''.<ref>{{citation
 | last1 = Coxeter | first1 = H.S.M. | author1-link = Harold Scott MacDonald Coxeter
 | last2 = Du Val | first2 = P.
 | last3 = Flather | first3 = H.T.
 | last4 = Petrie | first4 = J.F.
 | isbn = 978-1-899618-32-3
 | mr = 676126
 | orig-date = 1938
 | publisher = Tarquin Publications
 | title = The Fifty-Nine Icosahedra
 | title-link = The Fifty-Nine Icosahedra
 | year = 1999}}.</ref> Coxeter's analysis signalled a rebirth of interest in geometry. Coxeter himself went on to enumerate the star uniform polyhedra for the first time, to treat tilings of the plane as polyhedra, to discover the [[regular skew polyhedron|regular skew polyhedra]] and to develop the theory of [[complex polytope|complex polyhedra]] first discovered by Shephard in 1952, as well as making fundamental contributions to many other areas of geometry.<ref>{{citation|title=King of Infinite Space: Donald Coxeter, the Man Who Saved Geometry|first=Siobhan|last=Roberts|author-link=Siobhan Roberts|publisher=Bloomsbury Publishing|year=2009|isbn=9780802718327}}</ref>

In the second part of the twentieth century, both [[Branko Grünbaum]] and [[Imre Lakatos]] pointed out the tendency among mathematicians to define a "polyhedron" in different and sometimes incompatible ways to suit the needs of the moment.<ref name="lakatos"/><ref name=sin/> In a series of papers, Grünbaum broadened the accepted definition of a polyhedron, discovering many new [[Regular polyhedron#History|regular polyhedra]]. At the close of the 20th century these latter ideas merged with other work on incidence complexes to create the modern idea of an abstract polyhedron (as an abstract 3-polytope), notably presented by McMullen and Schulte.<ref>{{citation|last1=McMullen|first1=Peter|author1-link=Peter McMullen|last2=Schulte|first2=Egon|author2-link=Egon Schulte|title=Abstract Regular Polytopes|series=Encyclopedia of Mathematics and its Applications|volume=92|publisher=Cambridge University Press|year=2002}}</ref>

Polyhedra make a frequent appearance in modern [[computational geometry]], [[computer graphics]], and [[geometric design]] with topics including the reconstruction of polyhedral surfaces or [[Polygon mesh|surface meshes]] from scattered data points,<ref>{{citation
 | last1 = Lim | first1 = Seng Poh
 | last2 = Haron | first2 = Habibollah
 | date = March 2012
 | doi = 10.1007/s10462-012-9329-z
 | issue = 1
 | journal = Artificial Intelligence Review
 | pages = 59–78
 | title = Surface reconstruction techniques: a review
 | volume = 42| s2cid = 254232891
 }}</ref> geodesics on polyhedral surfaces,<ref>{{citation
 | last1 = Mitchell | first1 = Joseph S. B. | author1-link = Joseph S. B. Mitchell
 | last2 = Mount | first2 = David M. | author2-link = David Mount
 | last3 = Papadimitriou | first3 = Christos H. | author3-link = Christos Papadimitriou
 | doi = 10.1137/0216045
 | issue = 4
 | journal = [[SIAM Journal on Computing]]
 | mr = 899694
 | pages = 647–668
 | title = The discrete geodesic problem
 | volume = 16
 | year = 1987}}</ref> [[Visibility (geometry)|visibility]] and illumination in polyhedral scenes,<ref>{{citation
 | last1 = Teller | first1 = Seth J. | author1-link = Seth J. Teller
 | last2 = Hanrahan | first2 = Pat | author2-link = Pat Hanrahan
 | editor-last = Whitton | editor-first = Mary C. | editor-link = Mary Whitton
 | contribution = Global visibility algorithms for illumination computations
 | doi = 10.1145/166117.166148
 | pages = 239–246
 | publisher = Association for Computing Machinery
 | title = Proceedings of the 20th Annual Conference on Computer Graphics and Interactive Techniques, SIGGRAPH 1993, Anaheim, CA, USA, August 2–6, 1993
 | year = 1993| isbn = 0-89791-601-8 | s2cid = 7957200 }}</ref> [[polycube]]s and other non-convex polyhedra with axis-parallel sides,<ref>{{citation|title=Polycube Optimization and Applications: From the Digital World to Manufacturing|hdl=11584/261570|first=Gianmarco|last=Cherchi|date=February 2019|type=Doctoral dissertation|publisher=University of Cagliari}}</ref> algorithmic forms of Steinitz's theorem,<ref>{{citation
 | last = Rote | first = Günter
 | editor1-last = van Kreveld | editor1-first = Marc J.
 | editor2-last = Speckmann | editor2-first = Bettina
 | contribution = Realizing planar graphs as convex polytopes
 | doi = 10.1007/978-3-642-25878-7_23
 | pages = 238–241
 | publisher = Springer
 | series = Lecture Notes in Computer Science
 | title = Graph Drawing – 19th International Symposium, GD 2011, Eindhoven, The Netherlands, September 21–23, 2011, Revised Selected Papers
 | volume = 7034
 | year = 2011| doi-access = free
 | isbn = 978-3-642-25877-0
 }}</ref> and the still-unsolved problem of the existence of polyhedral nets for convex polyhedra.<ref>{{citation|last1=Demaine|first1=Erik|author1-link=Erik Demaine|last2=O'Rourke|first2=Joseph|author2-link=Joseph O'Rourke (professor)|title=Geometric Folding Algorithms: Linkages, Origami, Polyhedra|title-link=Geometric Folding Algorithms|publisher=Cambridge University Press|year=2007}}</ref>

==In nature==
<!--This needs a bit of writing, esp. re '''''crystals'''''.-->

For natural occurrences of regular polyhedra, see {{slink|Regular polyhedron|Regular polyhedra in nature}}.

Irregular polyhedra appear in nature as [[crystal]]s.

== See also ==
{{div col|colwidth=18em}}
* [[Extension of a polyhedron]]
* [[Goldberg polyhedron]]
* [[List of books about polyhedra]]
* [[List of convex regular-faced polyhedra]]
* [[List of small polyhedra by vertex count]]
* [[Near-miss Johnson solid]]
* [[Polyhedron model]]
* [[Polyhedral combinatorics]]
* [[Polyhedral group]]
* [[Polyhedral number]]
* [[Polyhedral skeletal electron pair theory]]
* [[Polyhedral space]]
* [[Polyhedral symbol]]
* [[Polyhedral terrain]]
* [[Polytope model]]
* [[Schlegel diagram]]
* [[Lists of shapes]]
* [[Stella (software)]]
{{div col end}}

== Notes ==
{{notelist|group="lower-alpha"}}

== References ==
{{reflist}}

== External links ==
{{Wiktionary|polyhedron}}
{{Commons|Polyhedra}}

=== General theory ===
* {{Mathworld | urlname=Polyhedron | title=Polyhedron | mode=cs2 }}
* [http://www.steelpillow.com/polyhedra/ Polyhedra Pages]
* [https://www.math.technion.ac.il/S/rl/docs/uniform.pdf Uniform Solution for Uniform Polyhedra by Dr. Zvi Har'El] {{Webarchive|url=https://web.archive.org/web/20151127053535/http://www.math.technion.ac.il/S/rl/docs/uniform.pdf |date=2015-11-27 }}
* [https://web.archive.org/web/20170224151555/http://www.uwgb.edu/dutchs/symmetry/symmetry.htm Symmetry, Crystals and Polyhedra]

=== Lists and databases of polyhedra ===
* [http://www.georgehart.com/virtual-polyhedra/vp.html Virtual Reality Polyhedra] – The Encyclopedia of Polyhedra.
* [http://www.eg-models.de/index.html Electronic Geometry Models] – Contains a peer reviewed selection of polyhedra with unusual properties.
* [http://www.orchidpalms.com/polyhedra/index.html Polyhedron Models] – Virtual polyhedra.
* [http://www.polyedergarten.de/ Paper Models of Uniform (and other) Polyhedra]

=== Free software ===
<!-- Pay-for resources should not be linked to, per WP:LINKSTOAVOID -->
* [https://web.archive.org/web/20170616023727/http://www.uff.br/cdme/pdp/pdp-html/pdp-en.html A Plethora of Polyhedra] – An interactive and free collection of polyhedra in Java. Features includes nets, planar sections, duals, truncations and stellations of more than 300 polyhedra.
* [http://dogfeathers.com/java/hyperstar.html Hyperspace Star Polytope Slicer] – Explorer java applet, includes a variety of 3d viewer options.
* [http://www.openscad.org/ openSCAD] – Free cross-platform software for programmers. Polyhedra are just one of the things you can model.  The [[b:OpenSCAD User Manual|openSCAD User Manual]] is also available.
* [http://www.openvolumemesh.org/ OpenVolumeMesh] – An open source cross-platform C++ library for handling polyhedral meshes. Developed by the Aachen Computer Graphics Group, RWTH Aachen University.
* [https://levskaya.github.com/polyhedronisme/ Polyhedronisme] {{Webarchive|url=https://web.archive.org/web/20120425234840/http://levskaya.github.com/polyhedronisme/ |date=2012-04-25 }} – Web-based tool for generating polyhedra models using [[Conway polyhedron notation|Conway Polyhedron Notation]].  Models can be exported as 2D PNG images, or as 3D OBJ or VRML2 files.
=== Resources for making physical models ===
<!-- Pay-for resources should not be linked to, per WP:LINKSTOAVOID -->
* [http://www.korthalsaltes.com/ Paper Models of Polyhedra] Free nets of polyhedra.
* [http://ldlewis.com/How-to-Build-Polyhedra/ Simple instructions for building over 30 paper polyhedra]
* [http://hbmeyer.de/flechten/indexeng.htm Polyhedra plaited with paper strips] – Polyhedra models constructed without use of glue.

{{Polyhedra}}
{{Polyhedron navigator}}
{{Authority control}}

[[Category:Polyhedra| ]]