An example is the English word which in the sentence "This is the house which Jack built." Here which Jack built is a relative clause modifying the noun house. The relative pronoun which marks the relative clause and refers (within the relative clause) to the man being referred to in the main clause. It can be considered to provide a link between the two sentences "This is a house" and "Jack built the house", where the house referred to in each case is the same.
In providing a link between a subordinate clause and a main clause, a relative pronoun is similar in function to a subordinating conjunction. Unlike a conjunction, however, a relative pronoun does not simply mark the subordinate (relative) clause, but also plays the role of a noun within that clause. For example, in the relative clause which Jack built given above, the pronoun which functions as the object of the verb build. 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.
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 of the main clause which the relative pronoun stands for within the relative clause (such as house in the above example) is called the antecedent of that pronoun. In most cases the antecedent is a noun or noun phrase, although it is also possible for the pronoun to 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 noun (the object of like) in the main clause. A relative pronoun used in this way is sometimes called a fused relative pronoun, since the antecedent appears to be 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 – these are described in detail in the article on relative clauses. English can also make relative clauses without relative pronouns in some cases, as in "The man you saw yesterday was my uncle", where 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, such as German, Serbo-Croatian and Latin, which have gender, number, and noun declensions, 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.
The words used as relative pronouns are often words which 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, do not have relative pronouns.
In English and German, 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. which).
With the relative pronouns, sentences (5) and (6) would read like this:
In sentences (7) and (8), the words which and who are the relative pronouns. The word which is used because the bank is a thing; the word who is used because the teller is a person. Alternatively, the relative pronoun that could be used in either case. For more details see English relative clauses.