A moment, viewed as a locus of event-particles, is represented by a linear equation in the four coordinates (
). But the converse is not true; namely, not every linear equation represents a moment. A pair of linear equations represent a level or a matrix, and three independent linear equations represent a rect or a point-track or a null-track.
49.6 If
and
be any two time-systems, two sets of mutually normal axes,
, and