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 focus of the book is on geometries having lines which are self-orthogonal, or even singular (orthogonal to all lines). The most significant examples concern the four-dimensional spacetime of special relativity, however no knowledge of physics is presumed. An initial chapter has been included which explains the physical interpretation of the different orthogonality relations. The mathematical background needed is basic abstract and linear algebra.