Dihedral angle

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.