In geometry, the pentagonal dipyramid (or bipyramid) is third of the infinite set of face-transitive dipyramids. The set of dipyramids is the dual of the uniform prisms. It is also a Johnson solid (J13), constructed of regular polygons. It can be seen as two pentagonal pyramids (J2) connected along their bases. As a Johnson solid, it is a convex deltahedron. Although it is face-transitive, it is not a Platonic solid because some vertices have four faces meeting and others have five faces. Image:Pentagonal dipyramid.pngThe Johnson solid 13 is made of 10 equilateral triangles.