An octahedron is a closed 3-d shape with 8 polygonal faces. There are 257 topologically different convex octahedra. The faces of octahedra can be triangles, quadrilaterals, pentagons, hexagons or heptagons. There are many examples of octahedra - a hexagonal prism, for example - which has no triangular face. At the other extreme, all the faces of a regular octahedron are equilateral triangles.
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