Thus the boundary of a duration consists of two momentary three-dimensional spaces.

An event will be said to 'occupy' the aggregate of event-particles which lie within it.
Two events which have 'junction' in the sense in which junction was described in my last lecture, and yet are separated so that neither event either overlaps or is part of the other event, are said to be 'adjoined.' This relation of adjunction issues in a peculiar relation between the boundaries of the two events.

The two boundaries must have a common portion which is in fact a continuous three-dimensional locus of event-particles in the four-dimensional manifold.
A three-dimensional locus of event-particles which is the common portion of the boundary of two adjoined events will be called a 'solid.' A solid may or may not lie completely in one moment.

A solid which does not lie in one moment will be called 'vagrant.' A solid which does lie in one moment will be called a volume.

A volume may be defined as the locus of the event-particles in which a moment intersects an event, provided that the two do intersect.