## Antiparallel

Two lines and are said to be antiparallel with respect to the sides of an Angle if they make the same angle in the opposite senses with the Bisector of that angle. If and are antiparallel with respect to and , then the latter are also antiparallel with respect to the former. Furthermore, if and are antiparallel, then the points , , , and are Concyclic (Johnson 1929, p. 172; Honsberger 1995, pp. 87-88).

Honsberger, R. Parallels and Antiparallels.'' §9.1 in Episodes in Nineteenth and Twentieth Century Euclidean Geometry. Washington, DC: Math. Assoc. Amer., pp. 87-88, 1995.