A decagon is a flat shape with exactly ten straight sides and ten corners (which mathematicians call vertices). The word comes from Greek: deca means ten and gon means angle.

Picture a stop sign, which has eight sides. A decagon has two more sides than that. If you drew one carefully with a ruler, you'd connect ten different points with ten straight lines, eventually closing the shape back where you started. Regular decagons, where all the sides and angles are equal, have a balanced, almost circular feeling to them. You might find decagons in tile patterns, architectural designs, or on certain coins from around the world.

Decagons belong to the family of shapes called polygons: two-dimensional figures made of straight sides. A triangle is a polygon with three sides, a quadrilateral has four, a pentagon has five, and so on. A decagon simply continues that pattern with ten sides instead.