ninesided

Ninesided is an adjective used to describe objects that have nine sides. In geometry, the standard name for a nine‑sided polygon is nonagon, with enneagon as an alternative form that reflects Greek roots. The term ninesided is more of a plain-language descriptor and is less common in formal mathematical texts.

A nine‑sided polygon has an interior angle sum of (9−2)×180 = 1260 degrees. If the polygon is regular, each interior angle measures 1260/9 = 140 degrees. While a regular nonagon is a valid geometric concept, it is not constructible with straightedge and compass using classical methods, since 9 is not of the form 2^k times the product of distinct Fermat primes.

In usage, ninesided describes the shape in everyday language, whereas nonagonal or enneagonal are preferred in technical writing. The term reflects a straightforward combination of the number nine and the word sided, and it serves as a practical descriptor when precision terminology is not required.

See also: nonagon, enneagon, polygon.