A flat 10-sided shape with sides of equal length is called a decagon. A solid 10-sided shape with faces of equal size and shape is called a decahedron.

Two-dimensional shapes are called polygons, which means they are flat, straight-sided and closed. Three-dimensional shapes are called polyhedrons. These are solid with flat faces. Polygons and polyhedrons with all sides or faces the same size are called regular and are named from the Greek words associated with the number of sides or faces they contain. Polygons and polyhedrons are two kinds of shapes studied in the branch of mathematics known as plane geometry.