 Convex polygon - Concave polygons: Encyclopedia II - Convex polygon - Concave polygonsIf a simple polygon is not convex, it is called concave. At least one internal angle of a concave polygon is larger than 180 degrees.
A concave polygon is often called re-entrant polygon (but in some cases the latter term has a different meaning).
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See also:Convex polygon, Convex polygon - Concave polygons Read more here: » Convex polygon: Encyclopedia II - Convex polygon - Concave polygons | 
