# dihedral angle

A dihedral angle is the angle defined by two planes or two faces of a polyhedron meeting
at an edge; for example, all the dihedral angles of a cube are 90°.
An almost-spherical polyhedron (with many faces) has small dihedral angles.

A dihedral angle is described by a point on one of the planes or faces,
the line of intersection (edge), and a point on the other plane. Should
a point *P* on one of the planes be so positioned that the line through
it perpendicular to the edge *E*,
intersects *E* at the same point as a line drawn perpendicular to *E* through a point *P'* on the other plane, then the angle *PEP'* is the **plane angle** of the dihedral angle.
All plane angles of a dihedral angle are equal.