Decagon

The decagon, or dec, is a polygon with 10 sides. A regular decagon has equal sides and equal angles. It has the most sides of any polygon that occurs as a face of a non-prismatic uniform polytope, although dodecagons appear in some tilings and hexadecagons appear in scaliform polychora.

The combining prefix is da-, as in dadip.

The only non-compound stellation of the decagon is the decagram. The only other polygons with a single non-compound stellation are the pentagon, the octagon, and the dodecagon.

It can be constructed as the uniform truncation of the regular pentagon.

Naming
The name decagon is derived from the Ancient Greek (10) and  (angle), referring to the number of vertices.

Other names include:


 * Dec, Bowers style acronym, short for "decagon".

Vertex coordinates
Coordinates for a decagon of unit edge length, centered at the origin are all sign changes of:


 * $$\left(±\frac12,\,±\frac{\sqrt{5+2\sqrt5}}{2}\right),$$
 * $$\left(±\frac{3+\sqrt5}{4},\,±\sqrt{\frac{5+\sqrt5}{8}}\right),$$
 * $$\left(±\frac{1+\sqrt5}{2},\,0\right).$$

Representations
A regular decagon can be represented by the following Coxeter diagrams:


 * x10o (regular),
 * x5x (H2 symmetry, generally a dipentagon),
 * to5ot&#zx (t=$\sqrt{(5+√5)/2}$, generally a pentambus),
 * xFV Tto&#zx (rectangular symmetry, t as above, T=ft),
 * xFVFx&#xt (axial edge-first),
 * otTTto&#xt (axial vertex-first).

Variations
Two main variants of the decagon have pentagon symmetry: the dipentagon, with two alternating side lengths and equal angles, and the dual pentambus, with two alternating angles and equal edges. Other less regular variations with rectangular, mirror, or no symmetry also exist.

Stellations

 * 1st stellation: Stellated decagon (compound of two pentagons)
 * 2nd stellation: Decagram
 * 3rd stellation: Stellated decagram (compound of two pentagrams)