A polyhedron of which two faces are congruent polygons in parallel planes, and the other faces are parallelograms (see illus.). The bases B are the congruent polygons; the lateral faces are the parallelograms; the lateral edges are the edges not lying in the bases; and the perpendicular distance between the bases is the altitude l. Sections parallel to the bases are congruent to the bases. A prism is a right prism if its lateral edges are perpendicular to the bases; an oblique prism otherwise. A prism is called a triangular prism if its bases are triangles; a pentagonal prism if its bases are pentagons; and a parallelepiped if its bases are parallelograms. The volume of any prism is equal to the area of its base times its altitude (V = Bh).