Devoted to a domain of plane geometry, defining not only concepts of incidence but also metric concepts such as orthogonality or reflection. Verifies the interrelationships of three theories showing how they are different representations of a single, unified theory. These include a purely geometric theory based on the concept of incidence structures with orthogonality or with reflections, mainly as a treatment of Euclidean and non-Euclidean planes and certain subplanes of these planes; a theory of three-dimensional metric vector spaces with their natural geometric interpretation; and a theory of special types of S-groups and their group planes.