This article is not about the legal concept of a restrictive covenant.

Should one use "that" or "which"? In syntax, the concepts of restrictive clause and non-restrictive clause can perhaps most easily be explained by contrasting two examples:

The Washington Monument, which is 555 feet tall, is the tallest building in Washington.

The house that was recently built is Senator Smith's house.

The clause "which is 555 feet tall", does not distinguish this "Washington Monument" from other "Washington Monuments", but the clause "that was recently built" does distinguish this house from other houses. That is what it means to say that the latter clause is restrictive and the former is not. Omitting a non-restrictive clause results in a meaningful and correct sentence; that is not true of restrictive clauses.

Some writers follow a normative rule that "which" should be used in non-restrictive clauses and "that" should be used in restrictive clauses. In this usage, the phrase

the house that is green

does not mean the same thing as

the house, which is green.

The former says which house is being referred to, the latter presupposes that that is already established.

Most writers do not adhere to this rule, perhaps considering it unnecessary to be explicit about the distinction.