Thus there are three types of parallelograms in the four-dimensional manifold of event-particles.
In parallelograms of the first type the two pairs of parallel sides are both of them pairs of rects.

In parallelograms of the second type one pair of parallel sides is a pair of rects and the other pair is a pair of point-tracks.

In parallelograms of the third type the two pairs of parallel sides are both of them pairs of point-tracks.
The first axiom of congruence is that the opposite sides of any parallelogram are congruent.

This axiom enables us to compare the lengths of any two segments either respectively on parallel rects or on the same rect.

Also it enables us to compare the lengths of any two segments either respectively on parallel point-tracks or on the same point-track.