A relative clause is a subordinate clause that modifies a noun. For example, the noun phrase the man who wasn't there contains the noun man, which is modified by the relative clause who wasn't there. In many languages, relative clauses are introduced by a special class of pronouns called relative pronouns; in the previous example, who is a relative pronoun. 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.
The antecedent of the relative clause (that is, the noun that is modified by it) can in theory be the subject of the main clause, or its object, or any other verb argument. However, many languages do not have the possibility, or a straightforward syntactic pattern, to relativise arguments other than the core ones (subject and direct object).
A relative clause follows the noun it modifies. It is generally indicated by a relative pronoun at the start of the clause, although sometimes simply by word order. The choice of relative pronoun, or choice to omit one, can be affected by whether the clause modifies a human or non-human noun, by whether the clause is restrictive or not, and by the role (subject, direct object, or the like) of the relative pronoun in the relative clause. In English, as in some other languages (such as French; see below), non-restrictive relative clauses are set off with commas, but restrictive ones are not:
* I met a man and a woman yesterday. The woman, who had a thick French accent, was very pretty.
* I met two women yesterday, one with a thick French accent and one with a mild German one. The woman who had a thick French accent was very pretty.
As regards relative clauses, English has two particularities that are unique among the Germanic languages:
1. In other Germanic languages, if a relative pronoun is the object of a preposition in the relative clause, then the preposition always appears at the start of the clause, before the relative pronoun. In English, the preposition will often appear where it would appear if the clause were an independent clause — in other words, the relative pronoun "strands" it when it moves to the start of the clause. It used to be common to regard this as a grammatical error (see: Linguistic prescription) but in fact it has been a standard feature of the language since Middle English times.
2. In other Germanic languages, a relative pronoun is always necessary. In English, however, it may be suppressed in a restrictive clause (as in "The man we met was very friendly"), provided it would not serve as the subject of the main verb. When this is done, if the relative clause is the object of a preposition in the relative clause, then said preposition is always "stranded" in the manner described above; it is never moved to the start of the clause.