 Though not historically the first, arguably the simplest way of expressing the idea of a strict conditional whose necessity varies depending on the antecedent can be found in Chellas (1975) and Segerberg (1989), who describe a conditional \(A&gt;B\) as a relative modality, namely a sententially indexed modality \(\Box_{A}B\), with the corresponding accessibility relation \(R_{\lvert A\rvert^{M}}\) for every sentence \(A\). A relative-modality frame or rm-frame can be defined as a pair \(\langle W, R\rangle\) where \(R\) is a ternary relation on \(W\times W\times \cP(W)\). In practice we can write \(R_Uvw\) instead of \(RvwU\), and see \(R\) as assigning a binary relation over worlds relative to a proposition \(U\). An rm-model is a triple \(M = \langle W, R, V\rangle\) such that \(\langle W, R\rangle\) is an rm-frame and \(V\) a valuation function. Given an rm-model \(M\), the truth condition for the conditional is this: