will be called normal to the rects and point-tracks in
. A pair of rects which are normal to each other will also be called 'perpendicular' or 'at right-angles.' Two point-tracks can never be normal to each other since no point-track lies on a level. Parallels to normals are themselves normal.
[47.2] Continuing the notation of [47.1] we note that the matrix
includes straight lines
, etc., of the spaces of
