More briefly, the figure may be defined as a **polyhedron** with two parallel faces containing all the vertices.

The points thus obtained are evidently the vertices of a **polyhedron** with plane faces.

If we take any **polyhedron** with plane faces, the null-planes of its vertices with respect to a given wrench will form another **polyhedron**, and the edges of the latter will be conjugate (in the above sense) to those of the former.

A **polyhedron** (A) is said to be the summital or facial holohedron of another (B) when the faces or vertices of A correspond to the edges of B, and the vertices or faces of A correspond to the vertices and faces together of B.

A **polyhedron** is said to be the hemihedral form of another **polyhedron** when its faces correspond to the alternate faces of the latter or holohedral form; consequently a hemihedral form has half the number of faces of the holohedral form.

The correspondence of the faces of polyhedra is also of importance, as may be seen from the manner in which one **polyhedron** may be derived from another.

The mensuration of the cube, and its relations to other geometrical solids are treated in the article **Polyhedron**; in the same article are treated the Archimedean solids, the truncated and snubcube; reference should be made to the article Crystallography for its significance as a crystal form.

If the figure be entirely to one side of any face the **polyhedron** is said to be " convex, " and it is obvious that the faces enwrap the centre once; if, on the other hand, the figure is to both sides of every face it is said to be concave, " and the centre is multiply enwrapped by the faces.

Take the pole of each face of such a **polyhedron** with respect to a paraboloid of revolution, these poles will be the vertices of a second **polyhedron** whose edges are the conjugate lines of those of the former.

Octahedra having triangular faces other than equilateral occur as crystal forms. See **Polyhedron** and Crystallography.

**POLYHEDRON** (Gr.