For example, the tunnel of a tube railway is an event at rest in a certain time-system, that is to say, it is cogredient with a certain duration.

A train travelling in it is part of that tunnel, but is not itself at rest.
If an event e be cogredient with a duration d, and d' be any duration which is part of d.

Then d' belongs to the same time-system as d.

Also d' intersects e in an event e' which is part of e and is cogredient with d'.
Let P be any event-particle lying in a given duration d.

Consider the aggregate of events in which P lies and which are also cogredient with d.