I love the New York Times!

When it comes to grammar, I'm a prescriptivist. Which is a good thing, given that I correct people's grammar for a living.

Confusion between "that" and "which" in restrictive and nonrestrictive clauses is one of my biggest pet peeves, and I was just over the moon to see it addressed in the New York Times' grammar blog.

After Deadline -- That? Which? Or What?

that, which. Use that, not which, in a restrictive clause — a clause necessary to the reader’s understanding of the sentence: The town that the pitcher calls home is tiny Hawley, Pa. (The sentence serves no purpose without that the pitcher calls home.) Note that there are no commas around the clause. In a nonrestrictive clause — one providing added information, not essential to understand the sentence — use which, preceded by a comma: Hawley, Pa., which the pitcher calls home, is tiny. (The sentence is understandable without which the pitcher calls home.)

This rule is often neglected in speech and colloquial writing, and some usage manuals take a laissez-faire approach (see, for example, the American Heritage Book of English Usage, which offers a good explanation but no firm guidance).

But preserving the distinction adds polish and helps to clarify the desired emphasis in a sentence.

(Restrictive and nonrestrictive clauses also show up in the practice tests for the UN editor exams, which, unfortunately, was the only way that I managed to convince some of my classmates that I didn't make this particular rule up out of thin air... So I love UN editors, too.)