In linguistics, verb valency or valence refers to the number of arguments controlled by a verbal predicate. It is related, though not identical, to verb transitivity, which counts only object arguments of the verbal predicate. Verb valency, on the other hand, includes all arguments, including the subject of the verb. The linguistic meaning of valence derives from the definition of valency in chemistry. This scientific metaphor is due to Lucien Tesnière, who developed verb valency into a major component of his (what would later become known as) dependency grammar theory of syntax and grammar. The notion of valency first appeared as a comprehensive concept in Tesnière's posthumously published book (1959) Éléments de syntaxe structurale (Elements of structural syntax)[1].
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There are several types of valency: impersonal (=avalent) , intransitive (=monovalent), transitive (=divalent) and ditransitive (=trivalent):
The term valence also refers to the syntactic category of these elements. Verbs show considerable variety in this respect. In the examples above, the arguments are noun phrases (NPs). But arguments can in many cases be other categories, e.g.
Many of these patterns can appear in a form rather different from the ones just shown above. For example, they can also be expressed using the passive voice:
The above examples show some of the most common valence patterns in English, but do not begin to exhaust them. Other linguists have examined the patterns of more than three thousand verbs and place them in one or more of several dozen groups ([2]).
The verb requires all of its arguments in a well-formed sentence, although they can sometimes undergo valency reduction or expansion. For instance, to eat is naturally divalent, as in he eats an apple, but may be reduced to monovalency in he eats. This is called valency reduction. In the southeastern United States, an emphatic trivalent form of eat is in use, as in I'll eat myself some supper. Verbs that are usually monovalent, like sleep, cannot take a direct object. However, there are cases where the valency of such verbs can be expanded, for instance in He sleeps the sleep of death. This is called valency expansion. Verb valence can also be described in terms of syntactic versus semantic criteria. The syntactic valency of a verb refers to the number of dependent arguments that the verb can have, while semantic valence describes the thematic relations associated with a verb.
Tesnière 1959[3] expresses the idea of valence as follows (translation from French):
Tesnière used the word actants to mean what are now widely called arguments (and sometimes complements). An important aspect of Tesnière's understanding of valency was that the subject is an actant (=argument, complement) of the verb in the same manner that the object is. The concept of subcategorization, which is related to valency but associated more with phrase structure grammars than with the dependency grammar that Tesnière developed, did not originally view the subject as part of the subcategorization frame[4], although the more modern understanding of subcategorization seems to be almost synonymous with valency.
Valence plays an important role in a number of the syntactic frameworks that have been developed in the last few decades. In Generalized Phrase Structure Grammar[5], many of the phrase structure rules generate the class of verbs with a particular valence. For example, the following rule generates the class of transitive verbs:
H stands for the head of the VP, that is the part which shares the same category as the VP, in this case, the verb. Some linguists objected that there would be one such rule for every valence pattern. Such a list would miss the fact that all such rules have certain properties in common. Work in Government and Binding[6] takes the approach of generating all such structures with a single schema, called the X-bar schema[7]:
X and Y can stand for a number of different lexical categories, and each instance of the symbol ’ stands for a bar. So A’, for instance, would be a kind of AP (adjective phrase). Two bars, used here for the complements, is thought by some linguists to be a maximal projection of a lexical category. Such a schema is meant to be combined with specific lexical rules and the projection principle to distinguish the various patterns of specific verbs.
Head-Driven Phrase Structure Grammar[8] introduces a handful of such schemata which aim to subsume all such valence related rules as well as other rules not related to valence. A network is developed for information related to specific lexical items. The network and one of the schemata aims to subsume the large number of specific rules defining the valence of particular lexical items.
Notice that the rule (VP → H NP [love]) and the schema (X’ → X, Y’’…) deal only with non-subject complements. This is because all of the above syntactic frameworks use a totally separate rule (or schema) to introduce the subject. This is a major difference between them and Tesnière's original understanding of valency, which included the subject, as mentioned above.
One of the most widely known versions of Construction Grammar [9] also treats the subject like other complements, but this may be because the emphasis is more on semantic roles and compatibility with work in cognitive science than on syntax.