A **parallelopiped** is a 3-D object, each of whose faces is a parallelogram.

A **rectangular parallelopiped** is the one, whose faces are rectangular.

A **cube** is a rectangular parallelopiped, whose edges are of equal length.

A **tetrahedron** is a 3-D object, whose all faces are triangular. It can be shown that a parallelopiped can be decomposed into 6 tetrahedra.

For a visual proof : **1 Parallelopipded = 6 Tetrahedra**

Let there be 2 such objects, whose coterminus edges are identical. Then, the volume of the tetrahedron is th of that of the parallelopiped. The expression can also be obtained using the following theorem :