 Possible worlds semantics  is celebrated for providing suitable interpretations for different axiomatic systems of  modal logic,  such as  C.I. Lewis’s  systems \(\mathbf{S4}\) and \(\mathbf{S5}\). In each model, sentences are evaluated as true or false relative to a possible world. Modal sentences \(\Box A\) and \(\Diamond A\) (usually read as ‘it is necessary that \(A\)’ and ‘it is possible that \(A\)’, respectively) are evaluated in terms of \(A\)’s truth at all or some of the accessible worlds. By placing various conditions on the accessibility relation between worlds, different modal logics can be accommodated. (Readers unfamiliar with modal logic are advised again to read the entities on  possible worlds  and  modal logic  before going further in this section.)