dihedral angle

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.

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