on", i.e., along their line of intersection . The dihedral angle \varphi_{AB} between two planes denoted A and B is the angle between their two normal unit vectors \mathbf{n}_{A} and \mathbf{n}_{B}: \cos \varphi_{AB} = \mathbf{n}_A \cdot \mathbf{n}_B. \, A dihedral angle can be signed ; for example, the dihedral angle \varphi_{AB}