Octahedron

The octahedron, or oct, is one of the five Platonic solids. It consists of 8 equilateral triangles, joined 4 to a square vertex. It is the 3 dimensional orthoplex. It also has 3 square pseudofaces. In fact, it can be built by joining two square pyramids by their square face, which makes it the square tegum.

It can also be constructed by rectifying the tetrahedron.

It is also the uniform triangular antiprism, and is a segmentohedron in this form.

It occurs as cells in one regular polychoron, namely the icositetrachoron.

Vertex coordinates
An octahedron of side length 1 has vertex coordinates given by all permutations of:
 * $$\left(±\frac{\sqrt2}{2},\,0,\,0\right).$$

Representations
A regular octahedron can be represented by the following Coxeter diagrams:


 * o4o3x (regular)
 * o3x3o (A3 symmetry, tetratetrahedron)
 * s2s3s (generally a triangular antiprism)
 * s2s6o (similar to above, as alternated hexagonal prism)
 * xo3ox&#x (A2 axial, generally a triangular antipodium)
 * oxo4ooo&#xt (BC2 axial, generally a square bipyramid)
 * oxo oxo&#xt (generally a rectangular bipyramid)
 * xox oqo&#xt (A1×A1 axial, edge-first)
 * oxox&#xr (single symmetry axis only)
 * qo ox4oo&#xt (BC2 prism symmetry square bipyramid)
 * qo ox ox&#xt (brick symmetry rectangle bipyramid)
 * qoo oqo ooq&#zx (brick symmetry, rhombic bipyramid)

Variations
Other variants of the octahedron exist, using 8 triangular faces with 6 4-fold vertices. Some of these include:


 * Tetratetrahedron - 2 sets of 4 triangles - just a coloring with no true variants in measures
 * Triangular antiprism - 2 equilateral bases, 6 isosceles sides, vertex transitive
 * Triangular antipodium - as above with 2 different sized bases and 2 sets of 3 isosceles sides
 * Square tegum - 8 isosceles triangles, square prism symmetry
 * Rectangular tegum - 2 sets of 4 isosceles triangles
 * Rhombic tegum - 8 scalene triangles, digonal prism symmetry
 * Digonal scalenohedron - 8 scalene triangles, digonal antiprism symmetry

Related polyhedra
The octahedron is the colonel of a two-member regiment that also includes the tetrahemihexahedron.

The octahedron is the regular-faced square bipyramid. If a cube, seen as a square prism, is inserted between the two haves, the result is an elongated square bipyramid.

A number of uniform polyhedron compounds are composed of octahedra, all but one of them featured octahedra in triangular antiprism symmetry:


 * Small icosicosahedron (5)
 * Snub octahedron (4)
 * Inner disnub octahedron (8, with rotational freedom)
 * Hexagrammic disnub octahedron (8)
 * Outer disnub octahedron (8, with rotational freedom)
 * Inner disnub tetrahedron (4, with rotational freedom)
 * Hexagrammic disnub tetrahedron (4)
 * Outer disnub tetrahedron (4, with rotational freedom)
 * Snub icosahedron (10)
 * Great snub icosahedron (10)
 * Outer disnub icosahedron (20, with rotational freedom)
 * Inner disnub icosahedron (20, with rotational freedom)
 * Great disnub icosahedron (20, with rotational freedom)
 * Disnub icosahedron (20)

There is also an infinite family of prismatic octahedron compounds, the antiprisms of compounds of triangles:
 * Hexagrammic antiprism (2)
 * Fissal enneagrammic antiprism (3)
 * Tetratriangular antiprism (4)

The octahedron has one stellation, the stella octangula.

The dihedral angle is a supplementary angle to that of the regular tetrahedron. Thus, augmenting one of the faces produces coplanar faces, disqualifying the resulting polyhedron from being a Johnson solid.