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