• Use "that" in restrictive clauses.

Example:"The person that stole my wallet is going to get a knuckle sandwich."

Because thephrase "that stole my wallet" restricts, or specifies, which personwill receive a knuckle sandwich, the clause is restrictive. When a clause isrestrictive, we generally begin the clause with the pronoun "that."

• Use "which" in nonrestrictive clauses.

Example:"The prison, which is filled with criminals, serves terrible food."

Because thephrase "which is filled with criminals" modifies the prison withoutspecifying or restricting the prison in question, we say the clause isnonrestrictive. When a clause is nonrestrictive, we generally begin the clausewith the pronoun "which."

（From: http://www.plasmodium.com）