Octahedron
Octahedron

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Type
 Platonic

Face polygon
 triangle

Faces
 8

Edges
 12

Vertices
 6

Faces per vertex
 4

Vertices per face
 3

Symmetry group
 octahedral (O_{h})

Dual polyhedron
 cube

Properties
 regular, convex

An
octahedron (plural: octahedra) is a polyhedron with eight faces. A
regular octahedron is a
Platonic solid composed of eight faces each of which is an equilateral triangle four of which meet at each vertex. The regular octahedron is a special kind of triangular
antiprism and of square
bipyramid, and is dual to the
cube. Canonical coordinates for the vertices of an octahedron centered at the origin are (±1,0,0), (0,±1,0), (0,0,±1).
The area A and the volume V of a regular octahedron of edge length a are:

The interior of the
compound of two dual
tetrahedra is an octahedron, and this compound, called the stella octangula, is its first and only
stellation. The vertices of the octahedron lie at the midpoints of the faces of the tetrahedron, and in this sense it relates to the tetrahedron in the same way that the
cuboctahedron and
icosidodecahedron relate to the other Platonic solids. One can also divide the edges of an octahedron in the ratio of the
golden mean to define the vertices of an
icosahedron. There are five octahedra that define any given icosahedron in this fashion, and together they define a regular compound.
Octahedra and tetrahedra can be mixed together to form a vertex, edge, and faceuniform tiling of space. This is the only such tiling save the regular tessellation of cubess, and is one of the five Andreini tessellations. Another is a tessellation of octahedra and cuboctahedra.
Using the standard nomenclature for Johnson solids, an octahedron would be called a square bipyramid.