  Another property which we might want for the relation ‘earlier than’ is density, the condition which says that between any two times we can always find another. Density would be false if time were atomic, i.e. if there were intervals of time which could not be broken down into any smaller parts. Density corresponds to the axiom \((C4): \Box \Box A\rightarrow \Box A\), the converse of (4), so for example, the system \(\mathbf{KC4}\), which is \(\bK\) plus \((C4)\) is adequate with respect to models where the frame \(\langle W, R\rangle\) is dense, and \(\mathbf{KDC4}\), adequate with respect to models whose frames are serial and dense, and so on.