The dihedral angle of two planes can be seen by looking at the planes "edge on", i.e., along their line of intersection. The dihedral angle AB between two planes denoted A and B is the angle between their two normal unit vectors A dihedral angle can be signed; for example, the dihedral angle AB can be defined as the angle through which plane A must be rotated to align it with plane B. Thus, AB = BA. For precision, one should specify the angle or its supplement, since both rotations will cause the planes to coincide.