\times \mathbf{b}_3], [\mathbf{b}_1 \times \mathbf{b}_2] \cdot [\mathbf{b}_2 \times \mathbf{b}_3] \right) where the two-argument atan2 takes care of the sign. Dihedral angles in polyhedra Every polyhedron, regular and irregular, convex and concave, has a dihedral angle at every edge. A dihedral angle (also called the face angle) is the internal angle at