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1911 Encyclopedia Britannica

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• Icosahedron
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1911 Encyclopedia Britannica
• Icosahedron

(Gr. eKovt, twenty, and g (3pa, a face or base), in geometry, a solid enclosed by twenty faces. The "regular icosahedron" is one of the Platonic solids; the "great icosahedron" is a Kepler-Poinsot solid; and the "truncated icosahedron" is an Archimedean solid (see Polyhedron). In crystallography the icosahedron is a possible form, but it has not been observed; it is closely simulated by a combination of the octahedron and pentagonal dodecahedron, which has twenty triangular faces, but only eight are equilateral, the remaining twelve being isosceles (see Crystallography).