A relative pronoun is a pronoun that marks a relative clause. An example is the word which in the sentence "This is the house which Jack built." Here the relative pronoun which introduces the relative clause, which modifies the noun house. A relative pronoun plays the role of a noun phrase within that clause. For example, in the relative clause "which Jack built," "which" is a pronoun functioning as the object of the verb "built."
In the English language, the following are the most common relative pronouns: which, who, whose, whom, whoever, whomever, and that, though some linguists analyze that in relative clauses as a conjunction / complementizer.
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).
Only about 7% of the languages around the world have relative pronouns. For example, Mandarin Chinese does not have relative pronouns at all and forms relative clauses (or their equivalents) by different methods.
Even within languages that have relative pronouns, not all relative clauses contain relative pronouns. For example, in the English sentence "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:
In some languages with gender, number, and noun declensions—such as German, Serbo-Croatian, Hindi, 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 some languages such as Hindi, the relative pronouns are distinct from the interrogative 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).
With the relative pronouns, sentences (1) and (2) would read like this:
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.