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Also ‑hedra and ‑hedral.

A geometrical solid with a specified number of plane faces.

Greek hedra, seat, base.

A tetrahedron is a solid having four plane triangular faces, a triangular pyramid; similarly, other solids with increasing numbers of plane faces have names that are usually based on Greek number prefixes (see the entry Number words): octahedron (eight), decahedron (ten), dodecahedron (twelve), icosahedron (twenty). In general, a polyhedron is any solid figure with many plane faces. A few terms relate to the shape of the faces rather than their number: rhombohedron, one with six equal rhombuses as faces; trapezohedron, one those faces are trapeziums or trapezoids. Plurals are traditionally formed in ‑hedra (polyhedra, tetrahedra), but now often in ‑s instead. Related adjectives are formed in ‑hedral (octahedral, dodecahedral). For flat geometric figures, see ‑gon.

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