that v. which

A relative clause, which describes a noun, can be either restrictive or non-restrictive. A restrictive clause contains essential (defining, identifying) information about the noun it is modifying. A non-restrictive clause contains non-essential (descriptive, additional) information.

• Restrictive: ‘The diner that serves amazing curly fries is open all night.’
• Non-restrictive: ‘Monk’s Cafe, which serves amazing curly fries, is open all night.’

In the restrictive example, ‘that serves amazing curly fries’ specifically identifies and limits which diner is being discussed. In the non-restrictive example, we already know that Monk’s Cafe, specifically, is being discussed; ‘which serves amazing curly fries’ just provides extra, descriptive information.

Use that before a restrictive clause, and which before a non-restrictive clause. A few practical cues can be used to make certain the right word is being used:

If the clause is essentially parenthetical, and can be omitted without changing the meaning of the rest of the sentence, use which; otherwise, use that. Which is the correct choice if it could be followed with, ‘(by the way)’.

• Correct: ‘Monk’s Cafe, which (by the way), serves amazing curly fries, is open all night.’
• Incorrect: ‘The diner that (by the way), serves amazing curly fries is open all night.’

Non-restrictive clauses are surrounded by commas; restrictive clauses aren’t.

Grammar Girl on that v. which

wiseGEEK on that v. which