Octadecagon

Regular octadecagon
A regular octadecagon
Type	general type of this shape
Edges and vertices	18
Schläfli symbol	{18} t{9}
Coxeter–Dynkin diagrams
Symmetry group	Dihedral (D₁₈)
Area	$\frac{18}{4}t^2 \cot \frac{\pi}{18}$ (with t = edge length)
Internal angle (degrees)	160°
Properties	convex, cyclic, equilateral, isogonal, isotoxal

An octadecagon is a polygon with 18 sides and 18 vertices. Another name for an octadecagon is octakaidecagon.

Construction

A regular octadecagon cannot be constructed using a compass and straightedge.

The regular octadecagon is the Petrie polygon for a number of higher dimensional polytopes, shown in these skew orthogonal projections from Coxeter planes:

Regular polygons

Listed by number of sides

1–10 sides	Henagon (Monogon) Digon Equilateral triangle Square Pentagon Hexagon Heptagon Octagon Nonagon (Enneagon) Decagon

11–20 sides	Hendecagon Dodecagon Triskaidecagon Tetradecagon Pentadecagon Hexadecagon Heptadecagon Octadecagon Nonadecagon (Enneadecagon) Icosagon

Others	Triacontagon Chiliagon Apeirogon

Star polygons	Pentagram Hexagram Heptagram Octagram Enneagram Decagram Hendecagram Dodecagram