A relative pronoun links two clauses into a single complex clause. To this extent, it is similar in function to a subordinating conjunction. Unlike a conjunction, however, a relative pronoun stands in place of a noun. Compare:
Sentence (2) consists of two clauses, a main clause (This is the house) and a relative clause (that Jack built). The word that is a relative pronoun. Within the relative clause, the relative pronoun stands for the noun phrase it references in the main clause (its antecedent), which is one of the arguments of the verb in the relative clause. In the example, the argument is the house, the direct object of built.
Other arguments can be relativised using relative pronouns:
Not all languages have relative pronouns. Those that do tend to use 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.
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/that). In some languages, the relative pronoun is an invariable word.
With the relative pronouns, sentences (5) and (6) would read like this:
In sentences (7) and (8), 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 'she' is a person.
like who and whose and more