Regular nonagon  

Edges and vertices  9 
Schläfli symbols  {9} 
Coxeter–Dynkin diagrams  
Symmetry group  Dihedral (D_{9}) 
Area (with t=edge length 

Internal angle (degrees) 
140° 
In geometry, a nonagon (or enneagon) is a ninesided polygon.
The name "nonagon" is a 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, though somewhat less common.
A regular nonagon has internal angles of 140°. The area of a regular nonagon of side length a is given by
Although a regular nonagon is not constructible with compass and straightedge there are methods of constructing that are very close approximations.
Graphs and stars
The K_{9} complete graph is often drawn as a regular nonagon with all 36 edges connected. This graph also represents an orthographic projection of the 9 vertices and 36 edges of the 8simplex.
The regular nonagon also contains 3 star figures, the third being the compound of three equilateral triangles:
Pop culture references
They Might Be Giants have a song entitled "Nonagon" on their children's album "Here Come the 123s." It refers to both an attendee at a party at which "everybody in the party is a manysided polygon" and a dance they perform at this party.[1]
References
 Weisstein, Eric W., "Nonagon" from MathWorld.
Graphs
The K_{9} complete graph is often drawn as a regular enneagon with all 36 edges connected. This graph also represents an orthographic projection of the 9 vertices and 36 edges of the 8simplex. Three other rectified forms also project inside of a regular enneagon:
8simplex (8D) 
Rectified 8simplex (8D) 
Birectified 8simplex (8D) 
Trirectified 8simplex (8D) 
External links
 Properties of a Nonagon (with interactive animation)

ar:تساعي (مضلع)
ast:Eneágonu
az:Düzgün doqquzbucaqlı
cs:Devítiúhelník
gl:Eneágono
it:Ennagono
ht:Nonagòn
hu:Kilencszög
nl:Negenhoek
no:Enneagon
nn:Enneagon
pl:Dziewięciokąt foremny
pt:Eneágono
sr:Деветоугао
sv:Nonagon
th:รูปเก้าเหลี่ยม