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Regular nonagon (enneagon)
Regular polygon 9 annotated.svg
A regular nonagon (enneagon)
Type Regular polygon
Edges and vertices 9
Schläfli symbol {9}
Coxeter diagram CDel node 1.pngCDel 9.pngCDel node.png
Symmetry group Dihedral (D9), order 2×9
Internal angle (degrees) 140°
Dual polygon self
Properties convex, cyclic, equilateral, isogonal, isotoxal

In geometry, a nonagon /ˈnɒnəɡɒn/ (or enneagon /ˈɛnəɡɒn/) is a nine-sided polygon.

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, though somewhat less common than "nonagon".

A regular nonagon has internal angles of 140°. The area of a regular nonagon of side length a is given by

A = \frac{9}{4}a^2\cot\frac{\pi}{9}\simeq6.18182\,a^2.


Although a regular nonagon is not constructible with compass and straightedge there are methods of construction that produce very close approximations. It can be constructed using neusis, or by allowing the use of an angle trisector.

See: File:Approximated Nonagon Inscribed in a Circle.gif for animation.


The K9 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 8-simplex.

8-simplex t0.svg
8-simplex (8D)

Pop culture references[edit]

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 many-sided polygon" and a dance they perform at this party.[1] Slipknot's logo is also a version of a nonagon, being a nine-pointed star made of 3 triangles.


Temples of Baha'i Faith are required to be nonagonal.

The U.S. Steel Tower is an irregular nonagon.

See also[edit]


  1. ^

External links[edit]