- This article covers restrictive clauses in grammar rather than the legal concept of a restrictive covenant.
Should one use "that" or "which"? In syntax, the concepts of restrictive clause and non-restrictive clause (or descriptive clause) 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 - the born yesterday resolves ambiguity - by identifying which of the two puppies is being referred to.
Given a group of puppies of various breeds, including a Dalmatian:
- The Dalmatian puppy, which was born yesterday, is tiny.
This is non-restrictive - The born yesterday provides additional information because the question of identity is already resolved, there being only one Dalmatian puppy.
A restrictive clause serves to disambiguate and is not preceded by a comma, whereas a non-restrictive clause simply adds descriptive information and is separated from its referent by a comma.
Some writers follow a normative rule that "which" should be used only in non-restrictive clauses and "that" should be used only in restrictive clauses.
According to this rule, to specify that one is talking about the green house in particular, one should write:
- the house that is green
Many writers do not adhere to this rule and use "which" for both types of clauses, largely because the comma used with descriptive but not with restrictive clauses is sufficient for disambiguation in writing.
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