# Dodecahedron

**DODECAHEDRON** , in geometry, a solid enclosed by twelve plane faces. The "ordinary dodecahedron" is one of the Platonic solids (see
Polyhedron). The Greeks discovered that if a line be divided in extreme and mean proportion, then the whole line and the greater segment are the lengths of the
edge of a cube and dodecahedron inscriptible in the same Sphere. The "small stellated dodecahedron," the "great dodecahedron" and the "great stellated
dodecahedron" are Kepler-Poinsot solids; and the "truncated" and "snub dodecahedra" are Archimedean solids (see Polyhedron). In crystallography, the regular or
ordinary dodecahedron is an impossible form since the faces cut the axes in irrational ratios; the "pentagonal dodecahedron" of crystallographers has irregular
pentagons for faces, while the geometrical solid, on the other hand, has regular ones. The "rhombic dodecahedron," one of the geometrical semiregular solids, is
an important crystal form. Many other dodecahedra exist as crystal forms, for which see *Crystallography*.

*Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)*