1911 Encyclopædia Britannica/Octahedron
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OCTAHEDRON (Gr. ὀκτώ, eight, ἕδρα, base), a solid bounded by eight triangular faces; it has 6 vertices and 12 edges. The regular octahedron has for its faces equilateral triangles; it is the reciprocal of the cube. Octahedra having triangular faces other than equilateral occur as crystal forms. See Polyhedron and Crystallography.