A point-track is a locus of event-particles.

It is defined by reference to one particular time-system, {alpha} say.

Corresponding to any other time-system these will be a different group of point-tracks.

Every event-particle will lie on one and only one point-track of the group belonging to any one time-system.

The group of point-tracks of the time-system {alpha} is the group of points of the timeless space of {alpha}.