Three-pointed star (concave hexagon). "In geometry, a polygon can be either convex or concave (non-convex or reentrant). ... Concave or non-convex polygons. A simple polygon that is not convex is called concave, non-convex or reentrant. A concave polygon will always have an interior angle with a measure that is greater than 180 degrees. It is always possible to partition a concave polygon into a set of convex polygons. A polynomial-time algorithm for finding a decomposition into as few convex polygons as possible is described by Chazelle & Dobkin (1985)." [Convex and concave polygons. Wikipedia]