'That' vs. 'which'

To fully understand when it is appropriate to write 'that' vs. 'which,' one must first grasp the distinction between restrictive and nonrestrictive clauses. While this might sound a rather dry task to some, anyone serious about writing must undertake it--and for me and a probably select group of others, it's a positive joy. So here it is: Restrictive clauses define the noun they modify and are essential to the meaning of the sentence, while nonrestrictive clauses describe the noun and may be removed without a change in meaning. A pair of examples will help illustrate the difference:

1. The cell phone that is stuck on Ted's roof is ringing. (restrictive)


2. My cell phone, which is on Ted's roof, is ringing. (nonrestrictive)

In the first sentence, the clause that is stuck on Ted's roof plays a defining role. As such, it is essential to the meaning of the sentence, and therefore could not be removed without dire semantic consequences. Without this clause, the sentence tells us that a cell phone is ringing--but it could be any cell phone, anywhere. It is the restrictive clause that narrows it down to just one, and thus admirably performs the defining function that any good restrictive clause fulfills.

The second sentence, by contrast, contains a nonrestrictive clause. Nonrestrictive clauses are, in essence, parenthetical--that is, they function as asides, and can be omitted without changing the meaning of the sentence. To be sure, they often provide additional, and important, information (it certainly matters whether my cell phone is on Ted's roof or in my pocket!)--but they are not needed to clarify the essential meaning of the sentence. Sentence two focuses on the fact that my cell phone is ringing; the fact that it is on Ted's roof is incidental, and does not help the reader decide which cell phone I'm talking about.

Got all that? OK--now, the easy part: 'that' vs. 'which.'

That is used in restrictive clauses.
Which is used in nonrestrictive ones.

In most cases, it's really that simple.
I must add a couple of caveats, however:
(1) Do note that this distinction is largely American, and that in Britain, it's probably about as important as the distinction between 'shall' and 'will' in America.
(2) As with most rules, there are occasionally exceptions--and cases that could go either way.