Bipyramid

A bipyramid is a polyhedron formed by joining two identical pyramids base-to-base. The bipyramids are the dual polyhedra of the prisms. Three bipyramids can be made out of all regular triangles, the octahedron (square bipyramid), which counts among the Platonic solids, and the triangular and pentagonal bipyramids, which count among the Johnson solids.