In more technical treatments of the geometry of polyhedra and higher-dimensional polytopes, the term is also used to mean an element of any dimension of a more general polytope (in any number of dimensions).
In elementary geometry, a face is a two-dimensional polygon on the boundary of a polyhedron. Other names for a polygonal face include side of a polyhedron, and tile of a Euclidean plane tessellation.
For example, any of the six squares that bound a cube is a face of the cube. Sometimes "face" is also used to refer to the 2-dimensional features of a 4-polytope. With this meaning, the 4-dimensional tesseract has 24 square faces, each sharing two of 8 cubic cells.
|Polyhedron||Star polyhedron||Euclidean tiling||Hyperbolic tiling||4-polytope|
The cube has 3 square faces per vertex.
The small stellated dodecahedron has 5 pentagrammic faces per vertex.
The square tiling in the Euclidean plane has 4 square faces per vertex.
The order-5 square tiling has 5 square faces per vertex.
The tesseract has 3 square faces per edge.
Some other polygons, which are not faces, are also important for polyhedra and tessellations. These include Petrie polygons, vertex figures and facets (flat polygons formed by coplanar vertices which do not lie in the same face of the polyhedron).
In higher-dimensional geometry the faces of a polytope are features of all dimensions. A face of dimension k is called a k-face. For example the polygonal faces of an ordinary polyhedron are 2-faces. In set theory, the set of faces of a polytope includes the polytope itself and the empty set where the empty set is for consistency given a "dimension" of −1. For any n-polytope (n-dimensional polytope), −1 ≤ k ≤ n.
For example, with this meaning, the faces of a cube include the empty set, its vertices (0-faces), edges (1-faces) and squares (2-faces), and the cube itself (3-face).
All of the following are the faces of a 4-dimensional polytope:
- 4-face – the 4-dimensional 4-polytope itself
- 3-faces – 3-dimensional cells (polyhedral faces)
- 2-faces – 2-dimensional faces (polygonal faces)
- 1-faces – 1-dimensional edges
- 0-faces – 0-dimensional vertices
- the empty set, which has dimension −1
In some areas of mathematics, such as polyhedral combinatorics, a polytope is by definition convex. Formally, a face of a polytope P is the intersection of P with any closed halfspace whose boundary is disjoint from the interior of P. From this definition it follows that the set of faces of a polytope includes the polytope itself and the empty set.
In other areas of mathematics, such as the theories of abstract polytopes and star polytopes, the requirement for convexity is relaxed. Abstract theory still requires that the set of faces include the polytope itself and the empty set.
Cell or 3-face
The tesseract has 3 cubic cells (3-faces) per edge.
The 120-cell has 3 dodecahedron cells (3-faces) per edge.
The cubic honeycomb fills Euclidean 3-space with cubes, with 4 cells (3-faces) per edge.
The order-4 dodecahedral honeycomb fills 3-dimensional hyperbolic space with dodecahedra, 4 cells (3-faces) per edge.
Facet or (n-1)-face
In higher-dimensional geometry, the facets of a n-polytope are the (n-1)-faces of dimension one less than the polytope itself. For example:
- The facets of a line segment are its 0-faces or vertices.
- The facets of a polygon are its 1-faces or edges.
- The facets of a polyhedron or plane tiling are its 2-faces.
- The facets of a 4-polytope or 3-honeycomb are its 3-faces.
- The facets of a 5-polytope or 4-honeycomb are its 4-faces.
Ridge or (n-2)-face
In related terminology, a (n − 2)-face of an n-polytope is called a ridge.
Peak or (n-3)-face
A (n − 3)-face of an n-polytope is called a peak.
- Merriam-Webster's Collegiate Dictionary (Eleventh ed.). Springfield, MA: Merriam-Webster. 2004.
- Matoušek, Jiří (2002), Lectures in Discrete Geometry, Graduate Texts in Mathematics 212, Springer, 5.3 Faces of a Convex Polytope, p. 86.
- Cromwell, Peter R. (1999), Polyhedra, Cambridge University Press, p. 13.
- Grünbaum, Branko (2003), Convex Polytopes, Graduate Texts in Mathematics 221 (2nd ed.), Springer, p. 17.
- Ziegler, Günter M. (1995), Lectures on Polytopes, Graduate Texts in Mathematics 152, Springer, Definition 2.1, p. 51.
- Matoušek (2002) and Ziegler (1995) use a slightly different but equivalent definition, which amounts to intersecting P with either a hyperplane disjoint from the interior of P or the whole space.
- Matoušek (2002), p. 87; Grünbaum (2003), p. 27; Ziegler (1995), p. 17.
- Matoušek (2002), p. 87; Ziegler (1995), p. 71.
- Olshevsky, George, Face at Glossary for Hyperspace.
- Olshevsky, George, Cell at Glossary for Hyperspace.
- Olshevsky, George, Ridge at Glossary for Hyperspace.
- Olshevsky, George, Peak at Glossary for Hyperspace.
- Weisstein, Eric W., "Face", MathWorld.
- Weisstein, Eric W., "Facet", MathWorld.
- Weisstein, Eric W., "Side", MathWorld.