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)

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