In chemistry, valency refers to the capacity of an atom or group of atoms to combine in specific proportions with other atoms or groups of atoms. The French linguist Lucien Tesnire is generally credited with introducing this term to linguistics, where it is used metaphorically for the capacity of a verb to combine with distinct arguments or valents (Crystal 1985). A verb like rain, which has no referential noun phrases associated with it, is said to be zero-place or avalent; a verb like disappear, which takes only a subject argument, is said to be one-place or monovalent; verbs like devour and give are said to be two-place (bivalent) and three-place (trivalent), respectively.

This chemical metaphor has had a pervasive influence in linguistics: causative and applicative morphemes are now described as ‘adding arguments’, while passives and middles are described as ‘suppressing’ or ‘deleting’ arguments, respectively. Entire sections of grammars are devoted to ‘valency-changing’, ‘valency-increasing’ or ‘valency-reducing’ processes, suggesting that the primary function of these grammatical processes is to regulate the number of arguments in clauses.

The chemical metaphor contrasts with an older tradition that distinguishes just two classes of predicates – Transitive and Intransitive – and a category of voice. Passive voice and middle voice are seen within this tradition as altering the ‘point of view’ or ‘centre of interest’ (Jesperson 1924: 167) within a clause rather than applying mathematical operations to it, and causatives and applicatives are sometimes included in, and sometimes excluded from, the traditional range of voice-related phenomena.