In geometry, a monogon, also known as a henagon, is a polygon with one edge and one vertex. It has Schläfli symbol {1}.^[1]

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Monogon
On a circle, a monogon is a tessellation with a single vertex, and one 360-degree arc edge.
Type	Regular polygon
Edges and vertices	1
Schläfli symbol	{1} or h{2}
Coxeter–Dynkin diagrams	or
Symmetry group	[ ], Cs
Dual polygon	Self-dual

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In Euclidean geometry a monogon is a degenerate polygon because its endpoints must coincide, unlike any Euclidean line segment. Most definitions of a polygon in Euclidean geometry do not admit the monogon.

In spherical geometry, a monogon can be constructed as a vertex on a great circle (equator). This forms a dihedron, {1,2}, with two hemispherical monogonal faces which share one 360° edge and one vertex. Its dual, a hosohedron, {2,1} has two antipodal vertices at the poles, one 360° lune face, and one edge (meridian) between the two vertices.^[1]

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