A relative clause is a clause that modifies a noun or noun phrase and uses some grammatical device to indicate that one of the arguments in the relative clause refers to the noun or noun phrase. For example, in the sentence I met a man who wasn't too sure of himself, the subordinate clause who wasn't too sure of himself is a relative clause since it modifies the noun man and uses the pronoun who to indicate that the same "MAN" is referred to in the subordinate clause (in this case as its subject).
In many languages, relative clauses are introduced by a special class of pronouns called relative pronouns, such as who in the example just given. In other languages, relative clauses may be marked in different ways: they may be introduced by a special class of conjunctions called relativizers, the main verb of the relative clause may appear in a special morphological variant, or a relative clause may be indicated by word order alone. In some languages, more than one of these mechanisms may be possible.
Types
Bound and free A bound relative clause, the type most often considered, qualifies an explicit element (usually a noun or noun phrase) appearing in the main clause, and refers back to that element by means of some explicit or implicit device within the relative clause.
The relative clause may also function as an embedded clause within a main (or higher-level) clause, thereby forming a matrix sentence. The noun in the main clause that the relative clause modifies is called the head noun, or (particularly when referred back to by a relative pronoun) the antecedent.
For example, in the English sentence "The person whom I saw yesterday went home", the relative clause "whom I saw yesterday" modifies the head noun person, and the relative pronoun whom refers back to the referent of that noun. The sentence is equivalent to the following two sentences: "I saw a person yesterday. That person went home". The shared argument need not fulfill the same role in both clauses; in this example the same person is referred to by the subject of the matrix clause, but the direct object of the relative clause.
A free relative clause (or fused relative), on the other hand, does not have an explicit antecedent external to itself. Instead, the relative clause itself takes the place of an argument in the matrix clause. For example, in the English sentence "I like what I see", the clause what I see is a free relative clause, because it has no antecedent, but itself serves as the object of the verb like in the main clause. Alternatively, one could argue that the free relative clause has a zero as its antecedent.
Restrictive and non-restrictive
Bound relative clauses may or may not be restrictive. A restrictive relative clause is a relative clause that functions as a restrictive modifier. A non-restrictive relative clause is a relative clause that is not a restrictive relative clause. 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). For example: The person who lives in this house has not been seen for days. This contains the restrictive relative clause who lives in this house, which modifies the meaning of person and is essential to the sentence. If this clause were omitted, it would no longer be known which person is being referred to, and the remaining part would not really make sense. The mayor, who lives in this house, has not been seen for days. This contains a non-restrictive relative clause since this provides supplementary information about the mayor but is not essential to the sentence. If this clause were omitted, it would still be known who is meant (the mayor), and the remaining part would still make 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). However, many languages distinguish the two types of relative clauses in this way only in speaking, not in writing. Another difference in English is that only restrictive relative clauses may be introduced with that or use the "zero" relative pronoun (see English relative clauses for details).
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, which refers not to the bed or the cat but to the entire proposition expressed in the main clause, namely the situation of the cat being allowed on the bed.
Formation methods Languages differ in many ways in how relative clauses are expressed: How the role of the shared noun phrase is indicated in the embedded clause. How the two clauses are joined together. Where the embedded clause is placed relative to the head noun (in the process indicating which noun phrase in the main clause is modified).
For example, the English sentence "The person that I saw yesterday went home" can be described as follows: The role of the shared noun in the embedded clause is indicated by gapping; that is, a gap is left in the object position after "saw", implying that the shared noun phrase ("the person") is to be understood to fill that gap and to serve as the object of the verb "saw". The clauses are joined by the complementizer "that". The embedded clause is placed after the head noun "the person".
The following sentences indicate various possibilities (only some of which are grammatical in English): "The person [that I saw yesterday] went home". (A complementizer linking the two clauses with a gapping strategy indicating the role of the shared noun in the embedded clause. One possibility in English. Very common cross-linguistically.) "The person [I saw yesterday] went home". (Gapping strategy, with no word joining the clauses—also known as a reduced relative clause. One possibility in English. Used in Arabic when the head noun is indefinite, as in "a person" instead of "the person".) "The person [whom I saw yesterday] went home". (A relative pronoun indicating the role of the shared noun in the embedded clause—in this case, the direct object. Used in formal English, as in Latin, German or Russian.) "The person [seen by me yesterday] went home". (A reduced relative clause, in this case passivized. One possibility in English.) "The person [that I saw him yesterday] went home". (A complementizer linking the two sentences with a resumptive pronoun indicating the role of the shared noun in the embedded clause, as in Arabic, Hebrew or Persian.) "The person [that him I saw yesterday] went home". (Similar to the previous, but with the resumptive pronoun fronted. This occurs in modern Greek and as one possibility in modern Hebrew; the combination that him of complementizer and resumptive pronoun behaves similarly to a unitary relative pronoun.) "The [I saw yesterday]'s person went home". (Preceding relative clause with gapping and use of a possessive particle—as normally used in a genitive construction—to link the relative clause to the head noun, as in Chinese.) "The [yesterday I seeing]'s person went home". (Preceding relative clause with gapping, and Nominalized the final verb, then use of a possessive particle—as normally used in a genitive construction—to link the relative clause to the head noun. This occurs in many Sino-Tibetan languages and possibly developed from "relative clause + noun" > "nominalized clause + noun" > "genitive construction"., as in Tibetan.) "The [I saw yesterday] person went home". (Preceding relative clause with gapping and no linking word, as in Japanese or Mongolian.) "The [yesterday my seeing] person went home". (Nominalized relative clause, as in Turkish.) Dün gördüğüm kişi eve gitti. "[Which person I saw yesterday], that person went home". (A correlative structure, as in Hindi.) "[I saw the person yesterday] went home." (An unreduced, internally headed relative clause, as in Navajo.)
Strategies for indicating the role of the shared noun in the relative clause There are four main strategies for indicating the role of the shared noun phrase in the embedded clause. These are typically listed in order of the degree to which the noun in the relative clause has been reduced, from most to least:
Gap strategy or gapped relative clause Relative pronoun Pronoun retention Nonreduction
Gapped relative clause In this strategy, there is simply a gap in the relative clause where the shared noun would go. This is normal in English, for example, and also in Chinese and Japanese. This is the most common type of relative clause, especially in verb-final languages with prenominal relative clauses, but is also widespread among languages with postnominal externally headed relative clauses.
There may or may not be any marker used to join the relative and main clauses. (Languages with a case-marked relative pronoun are technically not considered to employ the gapping strategy even though they do in fact have a gap, since the case of the relative pronoun indicates the role of the shared noun.) Often the form of the verb is different from that in main clauses and is to some degree nominalized, as in Turkish and in English reduced relative clauses.
In non-verb-final languages, apart from languages like Thai and Vietnamese with very strong politeness distinctions in their grammars, gapped relative clauses tend, however, to be restricted to positions high up in the accessibility hierarchy. With obliques and genitives, non-verb-final languages that do not have politeness restrictions on pronoun use tend to use pronoun retention. English is unusual in that all roles in the embedded clause can be indicated by gapping: e.g. "I saw the person who is my friend", but also (in progressively less accessible positions cross-linguistically, according to the accessibility hierarchy described below) "... who I know", "... who I gave a book to", "... who I spoke with", "... who I run slower than". Usually, languages with gapping disallow it beyond a certain level in the accessibility hierarchy, and switch to a different strategy at this point. Classical Arabic, for example, only allows gapping in the subject and sometimes the direct object; beyond that, a resumptive pronoun must be used. Some languages have no allowed strategies at all past a certain point—e.g. in many Austronesian languages, such as Tagalog, all relative clauses must have the shared noun serving the subject role in the embedded clause. In these languages, relative clauses with shared nouns serving "disallowed" roles can be expressed by passivizing the embedded sentence, thereby moving the noun in the embedded sentence into the subject position. This, for example, would transform "The person who I gave a book to" into "The person who was given a book by me". Generally, languages such as this "conspire" to implement general relativization by allowing passivization from all positions — hence a sentence equivalent to "The person who is run slower than by me" is grammatical. Gapping is often used in conjunction with case-marked relative pronouns (since the relative pronoun indicates the case role in the embedded clause), but this is not necessary (e.g. Chinese and Japanese both using gapping in conjunction with an indeclinable complementizer).
Relative pronoun type This is a type of gapped relative clause, but is distinguished by the fact that the role of the shared noun in the embedded clause is indicated indirectly by the case marking of the marker (the relative pronoun) used to join the main and embedded clauses. All languages which use relative pronouns have them in clause-initial position: though one could conceivably imagine a clause-final relative pronoun analogous to an adverbial subordinator in that position, they are unknown.