Each event-particle is contained in one and only one point of each time-system, and will be said to 'occupy' such a point. Two points of the same time-system never intersect; two point-tracks which are respectively points in the spaces of different time-systems either do not intersect or intersect in one event-particle only. Since each point-track intersects any moment in one and only one event-particle, two co-momental event-particles cannot lie on the same point-track. A pair of sequent event-particles lie in one and only one point-track, apart from exceptional cases when they lie in 'null-tracks.' Null-tracks are introduced later in article 45. 42.4 In the four-dimensional geometry of event-particles it has already been pointed out that rects have the character of straight lines, but that since sequent event-particles do not lie on the same rect there is a missing set of straight lines required to complete the geometry. Point-tracks [together with the exceptional set of loci termed 'null-tracks'] form this missing set of straight lines for this geometry of event-particles. The event-particles occupying a point-track have an order derived from the covering moments of any time-system. Those on a null-track have an order derived from routes which it is not necessary to discuss. 43. Parallelism. 43.1 A theory of parallelism holds for point-tracks and can be connected with the analogous theory for rects. Point-tracks which are points in the space of the same time-system are called 'parallel.' Thus a complete family of parallel point-tracks is merely a complete family of points in the space of some time-system. The parallelism of point-tracks is evidently transitive, symmetrical and reflexive. The definition of the parallelism of stations is derived from that of point-tracks. 43.2 The parallelism of point-tracks and the parallelism of rects and moments are interconnected. Let be any rect in a moment , and let be any family of parallel point-tracks. Then a certain set of point-tracks belonging to will intersect , and this set will intersect any moment parallel to in a rect parallel to . Again let be any point-track and let be any complete family of parallel rects. Then a certain set of rects belonging to will intersect ; name it . Let be any event-particle on some member of ; then the point-track containing and parallel to will intersect every member of . 43.3 A theorem analogous to those of 43.2 also holds for two families of point-tracks. Let be any point-track and let be any family of parallel point-tracks to which does not belong. Then a certain set of point-tracks belonging to will intersect ; name it . Let P be any event-particle occupying some member of ; then the point-track occupied by and parallel to will intersect every member of .