  Like ordinary modal logic, both these ‘pure’ calculi are monomodal; that is, each contains only one undefined modal operator. Prior wanted a ‘full tense calculus’, containing the two undefined operators \(G\) and \(H\), together with the operators \(F\) and \(P\) defined in terms of them (such a calculus is known as a \(bi\)modal logic). To obtain the full calculus it was not enough simply to bundle together the two ‘pure’ systems, because the two tense operators would remain independent of one another. Some interactive axioms, ‘laws which relate to the interaction of pastness and futurity’, were also required. Prior chose \(p \rightarrow\) \(GPp\) and \(p \rightarrow HFp.\)