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In geometry, a nonagon /ˈɒəɡɒ/ (or enneagon /ˈɛiːəɡɒ/) 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.
A regular nonagon has internal angles of 140°. The area of a regular nonagon of side length is given by
Construction
Although a regular nonagon is not constructible with compass and straightedge there are methods of construction that produce very close approximations.
Below is a close approximation of a regular nonagon. The approximate angle error is given in the animation.
Graphs
The K 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. Three other rectified forms also project inside of a regular enneagon:
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 many-sided polygon" and a dance they perform at this party.