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* In the [[alethic moods|alethic]] [[modal logic]] of [[Clarence Irving Lewis|C.I. Lewis]], the modal operator <math>\Box</math> expresses necessity: if the proposition ''A'' is read as "it is true that ''A'' holds", the proposition <math>\Box</math>''A'' is read is "it is ''necessarily'' true that ''A'' holds".
* In the [[tense logic]] (more commonly now called [[temporal logic]]) of [[Arthur Prior|A.N. Prior]], the proposition ''A'' is read as "''A'' is true at the present time"; '''F''' ''A'', as "''A'' will be true at some time in the future"; and '''G''' ''A'', as "''A'' is true now and will always be true".
* The previous two examples are of [[unary]] or [[Monad (category theory)|monadic]] modal operators. As an example of a [[dyadic]] modal operator -- which produces a new proposition from ''two'' old propositions -- is the operator '''P''' in the dyadic [[deontic logic]] of [[Georg Henrik von Wright|G.H. von Wright]]. '''P'''(''A'',''B'') expresses that "''A'' is obligatory under the circumstances ''B''".
{{Mathlogic-stub}}
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