 Any of various free modal or tense logics can be formalized by adding to a language \(\bL\) of the sort defined in  Section 2  the sentential operator ‘\(\Box\)’. If \(A\) is a formula, so is \(\Box A\). In alethic modal logic, this operator is read “it is necessarily the case that.” More generally, it means “it is true in all accessible worlds that,” where accessibililty from a given world is a different relation for different modalities: possibility for alethic logics, permissibility for deontic logics, various temporal relations for tense logics, and so on. A typical bivalent Kripke model \(\bM\) for such a language consists of a set of worlds, a binary accessibility relation \(\bR\) defined on that set; an assignment to each world \(w\) of a domain \(\bD_w\); an “outer” domain \(\bD_o\) of objects (which typically is either \(\bU\) or a superset thereof); and a two-place interpretation function \(\bI\) that assigns denotations at worlds to individual constants and extensions at worlds to predicates. For each individual constant \(t\) and world \(w, \bI(t,w)\in \bD_o\). In such a model, a singular term is a rigid designator if and only if for all worlds \(w_1\) and \(w_2\), \(\bI(t,w_1) = \bI(t,w_2)\). For every \(n\)-place predicate \(P, \bI(P,w) \subseteq \bD_w^n\) if the semantics is negative or neutral; if it is positive, \(\bI(P,w) \subseteq \bD_o^n\). Truth values at the worlds of a model \(\bM\) are assigned by a two-place valuation function \(\bV\) (where \(\bV(A,w)\) is read “the truth value \(\bV\) assigns to formula \(A\) at world \(w\)”) as follows: