In syntax, the concept of "restrictiveness" applies to a variety of syntactical constructions. Language learners most commonly encounter it in the context of relative clauses, where it is particularly problematic for learners of English, but linguists use the concept in a broader range of contexts. The first meaning of the term syntax, originating from the Greek words συν (sun, meaning ‘together’) and ταξις (taxis, meaning sequence/order), can be described as the study of the rules, or patterned relations that govern the way the words in a sentence come together. ... A relative clause is a subordinate clause that modifies a noun. ...

A number of different terms may be used. Most linguists speak of restrictive and non-restrictive clauses, but textbooks for non-specialists are more likely to refer to defining and non-defining relative clauses. Others again speak of integrated clauses and supplementary clauses, or of identifying and non-identifying relative clauses. Or the non-restrictive clause may be called a descriptive clause. Foreign learners of English are sometimes taught to call them necessary and unnecessary clauses, but this is easily misunderstood, as the information in an "unnecessary" clause may be necessary on other levels of importance.

The concept can perhaps most easily be explained by contrasting two examples:

• Given a litter of two puppies, one born today and one yesterday:

The puppy that was born yesterday is tiny.

This is restrictive: that was born yesterday resolves ambiguity — by identifying to which of the two puppies one refers. The relative clause serves to disambiguate; it defines the antecedent, restricts its reference, and is thus integrated into the basic idea of the main clause and is necessary if the main clause is to have its intended meaning.

• Given a group of puppies of various breeds, including a Dalmatian:

The Dalmatian puppy, which was born yesterday, is tiny.

This is non-restrictive: which was born yesterday provides additional, supplementary information, almost as an afterthought. It does not define the antecedent, because the question of identity is already resolved, there being only one Dalmatian puppy, but it may describe the antecedent.

In English, a restrictive clause is not preceded by a comma, whereas a non-restrictive clause is separated from its referent by a comma. Some writers follow a normative rule that that should be used only in restrictive clauses and which should be used only in non-restrictive clauses, in cases where the antecedent is a non-human; for humans, who and whom would be used, depending on grammatical case, and irrespective of whether the clause is restrictive or non-restrictive. However, many writers do not adhere to this rule, and in particular, which is widely used for non-humans, in both types of clauses. In linguistics, declension is a feature of inflected languages: generally, the alteration of a noun to indicate its grammatical role. ...

See also: Relative pronoun, Relative clause, English relative clauses A relative pronoun is a pronoun that introduces an adjective clause. ... A relative clause is a subordinate clause that modifies a noun. ... This article is focused mainly on usage of English relative clauses. ...