A relative clause is a kind of subordinate clause that contains the element whose interpretation is provided by an antecedent on which the subordinate clause is grammatically dependent; that is, there is an anaphoric relation between the relativized element in the relative clause and antecedent on which it depends.
Typically, a relative clause modifies a noun or noun phrase, and uses some grammatical device to indicate that one of the arguments within the relative clause has the same referent as that noun or noun phrase. For example, in the sentence *I met a man who wasn't there*, the subordinate clause who wasn't there is a relative clause, since it modifies the noun man, and uses the pronoun who to indicate that the same "man" is referred to within the subordinate clause (in this case, as its subject).
In many European languages, relative clauses are introduced by a special class of pronouns called relative pronouns, such as who in the example just given.
Restrictive and Non-restrictive
Bound relative clauses may or may not be restrictive. Whereas a non-restrictive or non-defining relative clause merely provides supplementary information, a restrictive or defining relative clause modifies the meaning of its head word (restricts its possible referent).
- *The man who lives in this house has not been seen for days. This (who lives in this house*) is a restrictive relative clause, modifying the meaning of man, and essential to the sentence (if the clause were omitted, it would no longer be known which man is being referred to). If the bold part is deleted the remaining part does not provide the sense.
- The mayor, who lives in this house, has not been seen for days. This is a non-restrictive relative clause, since it provides supplementary information about the mayor, but is not essential to the sentence – if the clause were omitted, it would still be known which mayor is meant. If the bold part is deleted the remaining part provides the sense. In speaking it is natural to make slight pauses around non-restrictive clauses, and in English this is shown in writing by commas (as in the examples).
Another difference in English is that only restrictive relative clauses may be introduced with that or use the "zero" relative pronoun. In colloquial speech, a non-restrictive relative clause may have a whole sentence as its antecedent rather than a specific noun phrase; for example:
- *The cat was allowed on the bed, which annoyed the dog*.
Here, the context of the sentence (presumably) indicates that which refers not to the bed or the cat but to the entire proposition expressed in the main clause, namely the circumstance that the cat was allowed on the bed. Such constructions are discouraged in formal usage and in texts written for nonnative speakers because of the potential for ambiguity in parsing; a construction more accepted in formal usage would be The cat's being [or having been] allowed on the bed annoyed the dog.
Finite and Non-Finite
Relative clauses may be either finite clauses (as in the examples above) or non-finite clauses. An example of a non-finite relative clause in English is the infinitive clause on whom to rely, in the sentence "She is the person on whom to rely".
Relative Clauses in English
In English, a relative clause follows the noun it modifies. It is generally indicated by a relative pronoun at the start of the clause, although sometimes simply by word order. If the relative pronoun is the object of the verb in the relative clause, it comes at the beginning of the clause even though it would come at the end of an independent clause ("He is the man whom I saw", not "He is the man I saw whom"). The choice of relative pronoun can be affected by whether the clause modifies a human or non-human noun, by whether the clause is restrictive or not, and by the role (subject, direct object, or the like) of the relative pronoun in the relative clause.
- For a human antecedent, "who", "whom", or "that" is usually used ("He is the person who saw me", "He is the person whom I saw", "He is the person that I saw"). For a non-human antecedent, only "that" or "which" is used.
- For a non-human antecedent in a non-restrictive clause, only "which" is used ("The tree, which fell, is over there"); while either "which" or "that" may be used in a restrictive clause ("The tree which fell is over there", "The tree that fell is over there")—but some styles and prescriptive grammars require the use of "that" in the restrictive context.
- Of the relative pronoun pair "who" and "whom", the subjective case form ("who") is used if it is the subject of the relative clause ("He is the policeman who saw me"); and, in formal usage, the objective case form ("whom") if it is the object of the verb or preposition in the relative clause ("He is the policeman whom I saw", "He is the policeman whom I talked to", "He is the policeman to whom I talked"); but in informal usage "whom" is often replaced by "who".
In English, non-restrictive relative clauses are set off with commas, but restrictive ones are not:
- "I met a man and a woman yesterday. The woman, who had a thick French accent, was very pretty." (non-restrictive—does not narrow down who is being talked about)
- "I met two women yesterday, one with a thick French accent and one with a mild Italian one. The woman who had the thick French accent was very pretty." (restrictive—adds information about who is being referred to)The status of "that" as a relative pronoun is not universally agreed.