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 :