| Regular enneagon (nonagon) | |
|---|---|
 A regular enneagon (nonagon) | |
| Type | Regular polygon |
| Edges and vertices | 9 |
| Schläfli symbol | {9} |
| Coxeter–Dynkin diagrams |    |
| Symmetry group | Dihedral (D9), order 2×9 |
| Internal angle (degrees) | 140° |
| Properties | Convex, cyclic, equilateral, isogonal, isotoxal |
| Dual polygon | Self |
In geometry, a nonagon (/ˈnɒnəɡɒn/) or enneagon (/ˈɛniəɡɒn/) is a nine-sided polygon or 9-gon.
The name nonagon is a prefix hybrid formation, from Latin (nonus, "ninth" + gonon), used equivalently, attested already in the 16th century in French nonogone and in English from the 17th century. The name enneagon comes from Greek enneagonon (εννεα, "nine" + γωνον (from γωνία = "corner")), and is arguably more correct,[1] though less common than "nonagon".