Regular hendecagon
Edges and vertices 11
Schläfli symbols {11}
Coxeter–Dynkin diagrams
Symmetry group Dihedral (D11)
Area
(with = edge length)
Internal angle
(degrees)
180°×(1-2/11)
=147°.27

In geometry, a hendecagon (also undecagon[1]) is an 11-sided polygon. The name "undecagon" is often seen as incorrect, but the matter is up for debate. The Greek prefix 'hen', is preferable to the Latin 'uni' or 'un'.[2] A regular hendecagon has internal angles of 147.27 degrees. The area of a regular hendecagon with side length is given by

A regular hendecagon is not constructible with compass and straightedge.

## Use in coinage

The Canadian dollar coin, the loonie, is patterned on a regular hendecagonal prism, as is the Indian two-rupee coin.

It was also patterned on the Susan B. Anthony dollar of the United States from 1979-1981 and again in 1999.

## Related shapes

The hendecagon shares the same set of 11 vertices with four regular hendecagrams, {11/2}, {11/3}, {11/4}, {11/5}.

## Petrie polygons

The regular hendecagon is the Petrie polygon for 10-dimensional uniform polytopes of the simplex family, projected in a skew orthogonal projection.[1][2]

 10-simplex Rectified 10-simplex Birectified 10-simplex Trirectified 10-simplex Quadrirectified 10-simplex