Polygons are two-dimensional shapes consisting of a cycle of line segments.
The segments are *edges* which meet in pairs at corners called *vertices*.
The picture shows several *regular* polygons, which have equal angles
and equal edge lengths. A polygon can be *self-intersecting,* meaning
edges cross other edges. (The points of intersection are not vertices.)

Regular polygons which are not self-intersecting are identified by an
integer corresponding to the number of sides (or vertices) it contains.
Regular self-intersecting polygons are identified by a fraction *n/m*,
where *n* is the number of edges (or vertices) and each vertex is
connected to the *m*th vertex around the circle of vertices.

A
polygon is *convex* if it is not self-intersecting and all interior
angles are less than 180 degrees. The simplest nonconvex regular polygon
is the *pentagram*, denoted 5/2.