In music, modality is the subject concerning certain diatonic scales known as "modes" (e.g., Ionian). See musical mode.
In language, modality is the subject concerning so-called "modal verbs" like can, must, and should, that are customarily used to modify the meaning of other verbs (which in turn tend to take the infinitive). Modal verbs express possibility (and impossibility, necessity, contingency, etc.), permissibility (and obligation, proscription, etc.), probability (and improbability, etc.).
In philosophy, modal logic is a form of logic which distinguishes between (logically) "necessary truths" and "contingent truths". Related topics are possibility, impossibility, actuality, and related predicates.
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