  (1) Obviously, we can unambiguously specify any ray in R3 through the origin by just giving one point contained in it. KS thus identify rays with points on the unit sphere E. KS do not need to refer to concrete coordinates of a certain point, since their argument is ‘coordinate-free’. We will, however, for illustration sometimes mention concrete points and then (a) use Cartesian coordinates to check orthogonality relations and (b) specify rays by points not lying on E. (Thus, e.g., the triple of points (0, 0, 1), (4, 1, 0), (1, –4, 0) is used to specify a triple of orthogonal rays.) Both usages conform with the recent literature (see e.g. Peres (1991) and Clifton (1993)).