Evidently each duration belongs to one and only one family of parallel durations; and thus each duration belongs to one and only one time-system. Also each moment belongs to one and only one family of parallel moments; and thus each moment belongs to one and only one time-system. Thus two distinct time-systems have no durations in common and no moments in common. But every event not a duration is contained in some durations of any given time-system. Furthermore there will be a minimum duration in a given time-system which is the duration 'when' the event happened in that time-system; namely, the minimum duration has the properties (i) that it extends over the event and (ii) that every duration which is part of it intersects the event. 34.3 The moments of a time-system are arranged in serial order in this way: (i) A duration belonging to a time-system is 'bounded' by a moment of the same time-system when each duration in which that moment inheres intersects the given duration and also intersects events separated from the given duration: (ii) Every duration has two such bounding moments, and every pair of parallel moments bound one duration of that time-system: (iii) A moment of a time-system 'lies between' two moments and of the same time-system when inheres in the duration which and bound: (iv) This relation of 'lying between' has the following properties which generate continuous serial order in each time-system, namely, () Of any three moments of the same time-system, one of them lies between the other two: () If the moment lies between the moments and , and the moment lies between the moments and , then lies between and : () There are not four moments in the same time-system such that one of them lies between each pair of the remaining three: () The serial-order among moments of the same time-system has the Cantor-Dedekind type of continuity. Nothing has yet been said about the measurement of the lapse of time. This topic will be considered as part of the general theory of congruence.