 By extending the list of axioms of \(\mathbf{K}_{t}\) with one or several of the above principles one can capture the validities of a number of natural models of time; that is, there are axiomatic extensions that are sound and complete for the corresponding classes of temporal frames. Some of these extensions were already being studied in the early days of Temporal Logic, and Prior (1967) discusses various systems obtained by postulating different combination of axioms.