In some linguistic theories, a relative pronoun is a term for a word that marks a type of subordinate clause - often called a relative clause, adjective clause or defining clause, among other names - as a means to conjoin modifying information about an antecedent referent in the main clause of a sentence.
An example is the word that in the sentence "This is the house that Jack built." Here the relative pronoun that conjoins the relative clause "Jack built," which modifies the noun house in the main sentence. That has an anaphoric relationship to its antecedent "house" in the main clause.
In the English language, the following are the most common relative pronouns: which, that, whose, whoever, whomever, who and whom.
In conjoining a subordinate clause and a main clause, a relative pronoun functions as a subordinating conjunction. According to some dependency grammar theories, a relative pronoun does not simply mark the subordinate (relative) clause but also may be considered to play the role of a noun within that clause. For example, in the relative clause "that Jack built" given above, the pronoun "that" may be construed to syntactically function as the object of the verb "built." Compare this with "Jack built the house after he married," where the conjunction after marks the subordinate clause after he married, but does not play the role of any noun within that clause.
In relative clauses, relative pronouns take the number (singular or plural) and the person (first, second or third) of its antecedent.
For more information on the formation and uses of relative clauses—with and without relative pronouns—see Relative clause. For detailed information about relative clauses and relative pronouns in English, see English relative clause.
The element in the main clause that the relative pronoun in the relative clause stands for (house in the above example) is the antecedent of that pronoun. In most cases the antecedent is a nominal (noun or noun phrase), though the pronoun can also refer to a whole proposition, as in "The train was late, which annoyed me greatly", where the antecedent of the relative pronoun which is the clause "The train was late" (the thing that annoyed me was the fact of the train's being late).
In a free relative clause, a relative pronoun has no antecedent: the relative clause itself plays the role of the co-referring element in the main clause. For example, in "I like what you did", what is a relative pronoun, but without an antecedent. The clause what you did itself plays the role of a nominal (the object of like) in the main clause. A relative pronoun used this way is sometimes called a fused relative pronoun, since the antecedent appears fused into the pronoun (what in this example can be regarded as a fusion of that which).
Not all relative clauses contain relative pronouns. Some languages, such as Mandarin Chinese, do not have relative pronouns at all, and form relative clauses (or their equivalents) by different methods. English can also make relative clauses without relative pronouns in some cases. For example, in "The man you saw yesterday was my uncle", the relative clause you saw yesterday contains no relative pronoun. It can be said to have a gap, or zero, in the position of the object of the verb saw.
Other arguments can be relativised using relative pronouns:
- Hunter is the boy who kissed Jessica.
- Indirect object
- Hunter is the boy to whom Jessica gave a gift./Hunter is the boy whom Jessica gave a gift to.
- Prepositional complement
- Jack built the house in which I now live. (Similarly with prepositions and prepositional phrases in general, for example, These are the walls between which Jack ran.)
- Jack is the boy whose friend built my house.
In some languages with gender, number, and noun declensions—such as German, Serbo-Croatian, and Latin—the relative pronoun agrees with its antecedent in gender and number, while its case indicates its relationship with the verb in the relative or main clause. In some other languages, the relative pronoun is an invariable word.
Words used as relative pronouns often originally had other functions. For example, the English which is also an interrogative word. This suggests that relative pronouns might be a fairly late development in many languages. Some languages, such as Welsh, have no relative pronouns.
In English, different pronouns are sometimes used if the antecedent is a human being, as opposed to a non-human or an inanimate object (as in who vs. that).
- (1) This is a bank. This bank accepted my identification.
- (2) She is a bank teller. She helped us open an account.
With the relative pronouns, sentences (1) and (2) would read like this:
- (3) This is the bank that accepted my identification.
- (4) She is the bank teller who helped us open an account.
In sentences (3) and (4), the words that and who are the relative pronouns. The word that is used because the bank is a thing; the word who is used because the teller is a person. Alternatively, which is often used in defining (or restrictive) relative clauses in either case. For details see English relative clauses.
- Gregory R. Guy and Robert Bayley, "On the Choice of Relative Pronouns in English", in American Speech: A Quarterly of Linguistic Usage, 70.2 (1995), pp. 148-62.
- Iliev, Iv., The Origin of Bulgarian Relative Pronouns. 
- Soojin Lee, "That or Which?: The that’s that of which is which, published in 2006, http://homes.chass.utoronto.ca/~cpercy/courses/6362-lee.htm
- Kordić, Snježana (1999). Der Relativsatz im Serbokroatischen [Relative Clauses in Serbo-Croatian]. Studies in Slavic Linguistics ; 10 (in German). Munich: Lincom Europa. p. 330. ISBN 3-89586-573-7. OCLC 42422661. OL 2863535W. Contents. Summary.