|Bowers style acronym||En|
|Coxeter diagram||x9o ()|
|Vertex figure||Dyad, length|
|Measures (edge length 1)|
|Number of external pieces||9|
|Level of complexity||1|
|Conjugates||Enneagram, great enneagram|
|Abstract & topological properties|
|Symmetry||I2(9), order 18|
The enneagon sometimes referred to as a nonagon, is a polygon with 9 sides. A regular enneagon has equal sides and equal angles.
The combining prefix in BSAs is e-, as in edip.
Like regular heptagons, regular enneagons are rarely found in higher polytopes that are objects of study, as they do not occur any non-prismatic uniform polyhedra or Johnson solids. A notable exception is the pairwise augmented cupolas, which are acrohedra. Enneagons also appear in some near-miss Johnson solids, such as the sesquitruncated octahedron.
The name enneagon is derived from the Ancient Greek ἐννέα (9) and γωνία (angle), referring to the number of vertices.
Other names include:
- En, Bowers style acronym, short for "enneagon".
Coordinates for an enneagon of edge length , centered at the origin, are:
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.
- 1st stellation: Enneagram
- 2nd stellation: Fissal enneagram (compound of three triangles)
- 3rd stellation: Great enneagram
- Bowers, Jonathan. "Regular Polygons and Other Two Dimensional Shapes".
- Wikipedia Contributors. "Nonagon".