In grammar and linguistics, the term complement is used with different meanings, so it is difficult to give a single precise definition and explanation. In a broad general sense however, a complement can be understood as a word, phrase or clause that is necessary to complete the meaning of a given expression. The terms complement and argument overlap in meaning and use. A given complement is therefore often also an argument. Complements are not adjuncts, however.
Predicative, subject and object complements
In many non-theoretical grammars, the terms subject complement and object complement are employed to denote the predicative expressions (e.g. predicative adjectives and nominals) that serve to assign a property to a subject or object, e.g.
- Ryan is upset. – Predicative adjective as subject complement
- Rachelle is the boss. – Predicative nominal as subject complement
- That made Michael lazy. – Predicative adjective as object complement
- We call Rachelle the boss. – Predicative nominal as object complement
This terminology is widespread in school grammar. It is retained in the important modern Comprehensive Grammar of Contemporary English to identify clause elements for labelling types of clause structure. Thus
|SV||intransitive||The sun is shining.||subject, verb|
|SVO||monotransitive||That lecture bored me.||subject, verb, object|
|SVC||copular||Your dinner seems ready.||subject, verb, subject complement|
|SVA||copular||My office is in the next building.||subject, verb, adverbial|
|SVOO||ditransitive||I must send my parents an anniversary card.||subject, verb, indirect object, direct object|
|SVOC||complex-transitive||Most students have found her reasonably helpful.||subject, verb, object, object complement|
|SVOA||complex-transitive||You can put the dish on the table.||subject, verb, object, adverbial|
However, this use of terminology is avoided by many modern theories of syntax, which typically view the expressions in bold as part of the clause predicate, which means they are not complements of the subject or object, but rather they are properties that are predicated of the subject or object.
The Cambridge Grammar of the English Language assigns the term "predicate complement" to both uses, and shifts the terminological distinction to the verb:
- Ed seemed quite competent: — complex-intransitive verb + predicative complement
- She considered Ed quite competent : — complex-transitive verb + predicative complement 
Complements as arguments
In many modern grammars (for instance in those that build on the X-bar framework), the object argument of a verbal predicate is called a complement. In fact, this use of the term is the one that currently dominates in linguistics. A main aspect of this understanding of complements is that the subject is usually NOT a complement of the predicate, e.g.
- He wiped the counter. – the counter is the object complement of the verb wiped.
- She scoured the tub. – the tub is the object complement of the verb scoured.
The noun phrases (NPs) the counter and the tub are necessary to complete the meaning of the verbs wiped and scoured, respectively; hence they are complements
While it is less common to do so, one sometimes extends this reasoning to subject arguments:
- He wiped the counter. – He is the subject complement of the verb wiped.
- She scoured the tub. – She is the subject complement of the verb scoured.
In these examples, the subject and object arguments are taken to be complements. In this area, then, the terms complement and argument overlap in meaning and use. Note that this practice takes a subject complement to be something very different from the subject complements of traditional grammar, which are predicative expressions, as just mentioned above.
Complements broadly construed
Construed in the broadest sense, any time a given expression is somehow necessary in order to render another expression "complete", it can be characterized as a complement of that expression, e.g.
- with the class – The NP the class is the complement of the preposition with.
- Jim will help. – The main verb help is the complement of the auxiliary verb will.
- Chris gave up. – The particle up is the complement of the verb gave.
- as a friend – The NP a friend is the complement of the particle of comparison as.
Construed in this broad sense, many complements cannot be understood as arguments. The argument concept is tied to the predicate concept in a way that the complement concept is not.
In linguistics, an adjunct is an optional, or structurally dispensable, part of a sentence, clause, or phrase that, when removed, will not affect the remainder of the sentence except to discard from it some auxiliary information. A more detailed definition of the adjunct emphasizes its attribute as a modifying form, word, or phrase that depends on another form, word, or phrase, being an element of clause structure with adverbial function. An adjunct is not an argument (nor is it a predicative expression), and an argument is not an adjunct. The argument-adjunct distinction is central in most theories of syntax and semantics. The terminology used to denote arguments and adjuncts can vary depending on the theory at hand. Some dependency grammars, for instance, employ the term circonstant (instead of adjunct), following Tesnière (1959).
- See Matthews (1981:142f.) and Huddleston (1988:note 2) for good overviews of the different uses of the term complement.
- See Crystal (1997:75).
- For examples of grammars that employ the terms subject complement and object complement to denote predicative expressions, see Matthews (1981:3ff.), Downing and Locke (1992:64f.), Thomas (1993:46, 49), Brinton (2000:183f.).
- Quirk et al. (1985) see especially pp 728-729
- Quirk et al (1985) p 721
- Huddlest and Pullum (2002)
- Huddleston and Pullum (2002) p 216
- For examples of this "narrow" understanding of complements, see for instance Lester (1971:83), Horrocks (1987:63), Borsley (1991:60ff.), Cowper (1992:67), Burton-Roberts (1997:41), Fromkin et al. (2000:119).
- For examples of theories that take the subject to be a complement of the matrix verb/predicate, see for instance Matthews (1981:101), Pollard and Sag (1994:23), Miller (2011:56).
- See Radford (2004:329) for an explanation of complements along these lines.
- Borsley, R. 1991. Syntactic theory: A unified approach. Cambridge, MA: Blackwell Publishers.
- Brinton, L. 2000. The structure of modern English. Amsterdam: John Benjamins Publishing Company.
- Burton-Roberts, N. 1997. Analysing sentences: An introduction to English grammar. London: Longman.
- Cowper, E. 1992. A concise introduction to syntactic theory: The government-binding approach. Chicago: The University of Chicago Press.
- Crystal, D. 1997. A dictionary of linguistics and phonetics, 4th edition, Oxford, UK: Blackwell.
- Downing, A. and P. Locke. 1992. English grammar: A university course, second edition. London: Routledge.
- Fromkin, V. et al. 2000. Linguistics: An introduction to linguistic theory. Malden, MA: Blackwell Publishers.
- Horrocks, G. 1986. Generative Grammar. Longman: London.
- Huddleston, R. 1988. English grammar: An outline. Cambridge, UK: Cambridge University Press.
- Huddleston, Rodney and Geoffrey K Pullum, 2002, The Cambridge Grammar of the English Language, Cambridge, Cambridge University Press. ISBN 0521431468
- Lester, M. 1971. Introductory transformational grammar of English. New York: Holt, Rinehart and Winston, Inc.
- Matthews, P. 1981. Syntax. Cambridge, UK: Cambridge University Press.
- Miller, J. 2011. A critical introduction to syntax. London: continuum.
- Pollard, C. and I. Sag. 1994. Head-Driven Phrase Structure Grammar. Chicago: The University Press of Chicago.
- Quirk, Raymond, Sidney Greembaum, Geoffrey Leech and Jan Scartvik, 1985, A Comprehensive Grammar of Contemporary English, Longman, London ISBN 0582517346.
- Radford, A. 2004. English syntax: An introduction. Cambridge, UK: Cambridge University Press.
- Thomas, L. 1993. Beginning syntax. Oxford, UK: Blackwell.