This book examines the geometrical notion of orthogonality, and shows how to use it as the primitive concept on which to base a metric structure in affine geometry. The subject has a long history, and an extensive literature, but whatever novelty there may be in the study presented here comes from its focus on geometries hav- ing lines that are self-orthogonal, or even singular (orthogonal to all lines). The most significant examples concern four-dimensional...