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.