are congruent to each other.
From the symmetry of normality either both pairs of particles, namely (
) and (
), are joined by rects, or both pairs are joined by point-tracks, or both pairs by null-tracks. As in the analogous case of congruence derived from parallelism, the transitiveness of congruence expresses a substantial law of nature and not a mere deduction from the terms of the definition.
Fig. 12.