Triangular dipyramid

For the related molecular geometry see trigonal bipyramid molecular geometry

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In geometry, the triangular dipyramid (or bipyramid) is the first in the infinite set of face-transitive dipyramids. It is the dual of the triangular prism with 6 isosceles triangle faces.

It is also one of the Johnson solids, (J₁₂) with equilateral triangle faces. As the name suggests, it can be constructed by joining two tetrahedra along one face. It is a convex deltahedron. Although all its faces are congruent and the solid is face-transitive, it is not a Platonic solid because some vertices adjoin three faces and others adjoin four. As a Johnson solid, with 6 equilateral triangles, it is also in the set of deltahedra.