In geometry, the orthobifastigium (digonal orthobicupola), is formed by gluing together two triangular prisms on their square faces, but without twisting. With regular faces, it has coplanar faces, so it is a limiting case of a Johnson solid. More generally the square can be isosceles trapezoids.

Orthobifastigium
Type	orthobicupola
Faces	4 triangles 4 trapezoids
Edges	14
Vertices	8
Symmetry group	D2h
Dual polyhedron	self-dual
Properties	convex

It is topologically a self-dual polyhedron and can also be called an elongated octahedron and self-dual octahedron.^[1]

These polyhedra resemble the dual gyrobifastigium in that both shapes have eight vertices and eight faces, with the faces forming a belt of four quadrilaterals separating two pairs of triangles from each other. However, in the dual gyrobifastigium the two pairs of triangles are twisted with respect to each other while in the orthobifastigium they are not.