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Regular icosagon
Regular polygon 20 annotated.svg
A regular icosagon
Type Regular polygon
Edges and vertices 20
Schläfli symbol {20}
Coxeter diagram CDel node 1.pngCDel 20.pngCDel node.png
Symmetry group D20, order 2×20
Internal angle (degrees) 162°
Dual polygon self
Properties convex, cyclic, equilateral, isogonal, isotoxal

In geometry, an icosagon is a twenty-sided polygon. The sum of any icosagon's interior angles is 3240 degrees.

One interior angle in a regular icosagon is 162°, meaning that one exterior angle would be 18°.

The regular icosagon is a constructible polygon, by an edge-bisection of a regular decagon, and can be seen as a truncated decagon.

Regular Icosagon[edit]

The area of a regular icosagon is: (with t = edge length)

A={5}t^2(1+\sqrt{5}+\sqrt{5+2\sqrt{5}}) \simeq 31.56875757 t^2.


The Big Wheel on the popular US game show The Price Is Right is an icosagon.

The Globe, the outdoor theater used by William Shakespeare's acting company, was discovered to have been built on an icosagonal foundation when a partial excavation was done in 1989.[1]

As a golygonal path, the swastika is considered to be an irregular icosagon.[2]

4.5.20 vertex.png A regular square, pentagon, and icosagon can completely fill a plane vertex.


A regular icosagon is constructible using a compass and straightedge:

Regular Icosagon Inscribed in a Circle.gif
Construction of a regular icosagon

Related polygons[edit]

A icosagram is a 20-sided star polygon. There are 3 regular forms given by Schläfli symbols: {20/3}, {20/7}, and {20/9}. There are also 5 regular star figures using the same vertex arrangement: 2{10}, 4{5}, 5{4}, 2{10/3}, and 4{5/2}.

Truncated polygon 20.svg
t{10} = {20}
Regular star polygon 20-3.svg
Regular star figure 2(10,1).svg
{20/2} = 2{10}
Regular star figure 4(5,2).svg
{20/4} = 4{5/2}
Regular star figure 5(5,1).svg
{20/5} = 5{5}
Regular star figure 2(10,3).svg
{20/6} = 2{10/3}
Regular star polygon 20-7.svg
Regular star figure 4(5,2).svg
{20/8} = 4{5/2}
Regular star polygon 20-9.svg

A regular icosagram, {20/9}, can be seen as a quasitruncated decagon, t{10/9}={20/9}. Similarly a decagram, {10/3} has a quasitruncation t{10/7}={20/7}, and finally a simple truncation of a decagram gives t{10/3}={20/3}

For each vertex-transitive geometric variations truncation can be constructed with two edge lengths.

Icosagrams as truncations of a regular decagons and decagrams, {10}, {10/3}
Regular polygon truncation 10 1.svg
Regular polygon truncation 10 2.svg Regular polygon truncation 10 3.svg Regular polygon truncation 10 4.svg Regular polygon truncation 10 5.svg Regular polygon truncation 10 6.svg
Regular star truncation 10-3 1.svg
Regular star truncation 10-3 2.svg Regular star truncation 10-3 3.svg Regular star truncation 10-3 4.svg Regular star truncation 10-3 5.svg Regular star truncation 10-3 6.svg

Petrie polygons[edit]

The regular icosagon is the Petrie polygon for a number of higher-dimensional polytopes, shown in orthogonal projections in Coxeter planes:

A19 B10 D11 E8 H4 2H2
19-simplex t0.svg
10-cube t9.svg
10-cube t0.svg
4 21 t0 p20.svg
600-cell t0 p20.svg
10-10 duopyramid ortho-3.png
10-10 duopyramid
10-10 duoprism ortho-3.png
10-10 duoprism