Also a family of parallel moments is the family of moments of some one time-system.

Thus we can enlarge our concept of a family of parallel levels so as to include levels in different moments of one time-system.

With this enlarged concept we say that a complete family of parallel levels in a time-system {alpha} is the complete family of levels in which the moments of {alpha} intersect the moments of {beta}.
This complete family of parallel levels is also evidently a family lying in the moments of the time-system {beta}.

By introducing a third time-system {gamma}, parallel rects are obtained.

Also all the points of any one time-system form a family of parallel point-tracks.