# Bipyramid

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Set of bipyramids

(Example hexagonal form)
Coxeter diagram Template:CDD
Schläfli symbol { } + {n}
Faces 2n triangles
Edges 3n
Vertices 2 + n
Face configuration V4.4.n
Symmetry group Dnh, [n,2], (*n22), order 4n
Rotation group Dn, [n,2]+, (n22), order 2n
Dual polyhedron n-gonal prism
Properties convex, face-transitive
Net
A bipyramid made with straws and elastics. An extra axial straw is added which doesn't exist in the simple polyhedron

An n-gonal bipyramid or dipyramid is a polyhedron formed by joining an n-gonal pyramid and its mirror image base-to-base.

The referenced n-gon in the name of the bipyramids is not an external face but an internal one, existing on the primary symmetry plane which connects the two pyramid halves.

The face-transitive bipyramids are the dual polyhedra of the uniform prisms and will generally have isosceles triangle faces.

A bipyramid can be projected on a sphere or globe as n equally spaced lines of longitude going from pole to pole, and bisected by a line around the equator.

Bipyramid faces, projected as spherical triangles, represent the fundamental domains in the dihedral symmetry Dnh.

## Volume

The volume of a bipyramid is ${\displaystyle \scriptstyle {V=}{\tfrac {2}{3}}\scriptstyle {Bh}}$ where B is the area of the base and h the height from the base to the apex. This works for any location of the apex, provided that h is measured as the perpendicular distance from the plane which contains the base.

The volume of a bipyramid whose base is a regular n-sided polygon with side length s and whose height is h is therefore:

${\displaystyle V={\frac {n}{6}}hs^{2}\cot {\frac {\pi }{n}}.}$

## Equilateral triangle bipyramids

Only three kinds of bipyramids can have all edges of the same length (which implies that all faces are equilateral triangles, and thus the bipyramid is a deltahedron): the triangular, tetragonal, and pentagonal bipyramids. The tetragonal bipyramid with identical edges, or regular octahedron, counts among the Platonic solids, while the triangular and pentagonal bipyramids with identical edges count among the Johnson solids (J12 and J13).

 Triangular bipyramid Square bipyramid(Octahedron) Pentagonal bipyramid

## Kalidescopic symmetry

If the base is regular and the line through the apexes intersects the base at its center, the symmetry group of the n-agonal bipyramid has dihedral symmetry Dnh of order 4n, except in the case of a regular octahedron, which has the larger octahedral symmetry group Oh of order 48, which has three versions of D4h as subgroups. The rotation group is Dn of order 2n, except in the case of a regular octahedron, which has the larger symmetry group O of order 24, which has three versions of D4 as subgroups.

The digonal faces of a spherical 2n-bipyramid represents the fundamental domains of dihedral symmetry in three dimensions: Dnh, [n,2], (*n22), order 4n. The reflection domains can be shown as alternately colored triangles as mirror images.

D1h D2h D3h D4h D5h D6h ...

## Star bipyramids

Self-intersecting bipyramids exist with a star polygon central figure, defined by triangular faces connecting each polygon edge to these two points. A {p/q} bipyramid has Coxeter diagram Template:CDD.

5/2 7/2 7/3 8/3 9/2 9/4 10/3 11/2 11/3 11/4 11/5 12/5

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## 4-polytopes with bipyramid cells

The dual of the rectification of each convex regular 4-polytopes is a cell-transitive 4-polytope with bipyramidal cells. In the following, the apex vertex of the bipyramid is A and an equator vertex is E. The distance between adjacent vertices on the equator EE=1, the apex to equator edge is AE and the distance between the apices is AA. The bipyramid 4-polytope will have VA vertices where the apices of NA bipyramids meet. It will have VE vertices where the type E vertices of NE bipyramids meet. NAE bipyramids meet along each type AE edge. NEE bipyramids meet along each type EE edge. CAE is the cosine of the dihedral angle along an AE edge. CEE is the cosine of the dihedral angle along an EE edge. As cells must fit around an edge, NAA cos−1(CAA) ≤ 2π, NAE cos−1(CAE) ≤ 2π.

*The rectified 16-cell is the regular 24-cell and vertices are all equivalent – octahedra are regular bipyramids. **Given numerically due to more complex form.

## Higher dimensions

In general, a bipyramid can be seen as an n-polytope constructed with a (n−1)-polytope in a hyperplane with two points in opposite directions, equal distance perpendicular from the hyperplane. If the (n−1)-polytope is a regular polytope, it will have identical pyramids facets. An example is the 16-cell, which is an octahedral bipyramid, and more generally an n-orthoplex is an (n-1)-orthoplex bypyramid.