Triangular tegum

The triangular bipyramid, or tridpy, also called a triangular dipyramid, is one of the 92 Johnson solids (J12). It has 6 equilateral triangles as faces, with 2 order-3 and 3 order-4 vertices. It can be constructed by joining two regular tetrahedra at one of their faces.

It is one of three regular polygonal bipyramids to be CRF. The others are the regular octahedron (square bipyramid) and the pentagonal bipyramid.

Vertex coordinates
A triangular bipyramid of edge length 1 has the following vertices:


 * $$\left(±\frac{1}{2},\,-\frac{\sqrt3}{6},\,0\right),$$
 * $$\left(0,\,\frac{\sqrt3}{3},\,0\right),$$
 * $$\left(0,\,0,\,±\frac{\sqrt6}{3}\right).$$

Representations
A triangular bipyramid has the following Coxeter diagrams:


 * oxo3ooo&#xt (as tower)
 * yo ox3oo&#zx (y = 2$\sqrt{6}$/3, as tegum product)
 * oyo oox&#xt (digonal symmetry)

Variations
The triangular bipyramid can have the height of its pyramids varied while maintaining its full symmetry These variations generally have 6 isosceles triangles for faces.

One notable variations can be obtained as the dual of the uniform triangular prism, which can be represented by m2m3o. In this variant the edges of the base triangle are exactly 1.5 times the length of the side edges, and all the dihedral angles are $$\arccos\left(-\frac17\right) ≈ 98.21321°$$. Each face has apex angle $$\arccos\left(-\frac18\right) \approx 97.18076°$$ and base angles $$\arccos\left(\frac34\right) \approx 41.40962°$$.

A triangular bipyramid with base edges of length b and side edges of length l has volume given by $$\frac{\sqrt3b^2}{6}\sqrt{l^2-\frac{b^2}{3}}$$.

Other triangular bipyramids
Besides this fully symmetric version, other 5-vertex polyhedra with 6 triangular faces exist:


 * Apiculated triangular pyramid - the two pyramids are different heights, dual to a triangular frustum
 * Notch - 2 isosceles and 4 scalene triangles, dual to a wedge
 * Scalene notch - 3 pairs of scalene triangles, dual to a skewed wedge

Related polyhedra
A triangular prism can be inserted between the halves of the triangular bipyramid to produce the elongated triangular bipyramid. Trying to insert a regular octahedron (as a triangular antiprism) would result in pairs of triangles becoming coplanar and turning into 60°/120° rhombi, resulting in a trigonal trapezohedron, a variant of the cube.