Enneagon

The enneagon or en, sometimes referred to as a nonagon, is a polygon with 9 sides. A regular enneagon has equal sides and equal angles.

The combining prefix is e-, as in edip.

Like heptagons, enneagons are generally rare in higher polytopes.

Naming
The name enneagon is derived from the Ancient Greek ' (9) and ' (angle), referring to the number of vertices.

Other names include:


 * En, Bowers style acronym, short for "enneagon".

Vertex coordinates
Coordinates for an enneagon of edge length 2sin(π/9), centered at the origin, are:


 * (1, 0),
 * (cos(2π/9), ±sin(2π/9)),
 * (cos(4π/9), ±sin(4π/9)),
 * (–1/2, ±$\sqrt{3}$/2),
 * (cos(8π/9), ±sin(8π/9)).

Variations
Besides the regular enneagon, other enneagons with triangular, mirror, or no symmetry exist. A few higher polytopes, such as certain swirlchora, have trigon-symmetric enneagons as facces.

Stellations

 * 1st stellation: Enneagram
 * 2nd stellation: Fissal enneagram (compound of three triangles)
 * 3rd stellation: Great enneagram