Diheral is the upward angle from horizontal in a fixed-wing aircraft or bird wing from root to tip, as viewed from directly in front or behind the aircraft. Dihedrals are also used in some types of kites such as box kites. The aerodynamic stabilising qualities of the dihedral were first described by Sir George Cayley in 1808/09. Downward angled wings have negative dihedral, or anhedral. Wings with local dihedral angles that change along the span are polyhedral.