A heptagon is a flat geometric shape with exactly seven straight sides and seven corners (called vertices). The word comes from Greek: hepta means seven and gon means angle.

You've probably drawn plenty of triangles (three sides), squares (four sides), and hexagons (six sides), but heptagons are less common in everyday life. They're trickier to construct with a compass and straightedge than some other polygons, which is partly why architects and designers use them less. The British 50-pence and 20-pence coins are based on heptagons, though with gently curved sides.

A regular heptagon has all seven sides the same length and all seven angles equal, making it perfectly symmetrical. An irregular heptagon still has seven sides, but they can be different lengths and the angles can vary. As long as a shape is flat, closed, and has exactly seven straight sides, it's a heptagon, whether those sides form a neat, balanced shape or a lopsided one.