An angle formed by the intersection of two planes (e.g. two faces of a polyhedron). The dihedral angle is the angle formed by taking a point on the line of intersection and drawing two lines from this point, one in each plane, perpendicular to the line of intersection. Because each set of three non-colinear atoms in a molecule defines a plane, the angle formed when two such molecular planes intersect is a dihedral angle (e.g. the angle made by the two planes formed by four atoms in a molecule in which the planes have two atoms in common). The concept of dihedral angle is used very extensively in the discussion of conformation and stereochemistry (see Ramachandran plot; torsion angle). The dihedral group, denoted Dn, is the group of symmetries of a regular polygon with n sides.