In geometry, the angle between two planes is called their dihedral or torsion angle.

[[Image:Sphericalbond_dihedral_angle.png|thumb|Figure 1: Dihedral angle of three vectors, defined as an exterior spherical angle. The longer and shorter black segments are arcs of the great circles passing through $mathbf\{b\}$ and $mathbf\{b\}${2} and through $mathbf\{b\}$ and $mathbf\{b\}${3}, respectively.]]

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 $phi$ between two planes denoted A and B is the angle between their two normal unit vectors $mathbf\{n\}${A} and $mathbf\{n\}$:

$cos\; phi${AB} = mathbf{n} cdot mathbf{n}{B}.

A dihedral angle can be _signed; for example, the dihedral angle $phi${AB} can be defined as the angle through which plane A must be rotated (about their common line of intersection) to align it with plane B. Thus, $phi$ = -phi_{BA}. For precision, one should specify the angle or its supplement, since both rotations will cause the planes to coincide.