In geometry, a set of lines in Euclidean space is called equiangular if every pair of lines makes the same angle.
Equiangular lines are related to two-graphs. Given a set of equiangular lines, let c be the cosine of the common angle. We assume that angle is not 90°, since that case is trivial (i.e., not interesting, because the lines are just coordinate axes); thus, c is nonzero. We may move the lines so they all pass through the origin of coordinates. Choose one unit vector in each line. Form the matrix M of inner products. This matrix has 1 on the diagonal and ±c everywhere else, and it is symmetric. Subtracting the identity matrix I and dividing by c, we have a symmetric matrix with zero diagonal and ±1 off the diagonal. This is the adjacency matrix of a two-graph.