[Note: A later and more complete post on this subject ran on April 28, 2010.]
Q: I was wondering if you could clear up something that’s bothering me. What’s the difference between “which” and “that”?
A: We think there’s a pretty good explanation in Pat’s book Woe Is I. When you’ve got a clause that you could start with “that” or “which” and you can’t decide between them, here’s a hint: If you can drop the information and not lose the point of the sentence, use “which.” If you can’t drop it, use “that.”
The examples in the book are: “Buster’s bulldog, which had one white ear, won best in show. The dog that won best in show was Buster’s bulldog.”
In the first example, the information in the “which” clause is not essential. In the second example, the clause starting with “that” is essential; it’s the whole point of the sentence.
You’ll also notice that “which” clauses are set off with commas, and “that” clauses aren’t. So if you find yourself wanting to insert little pauses before and after the information, it’s probably a “which” clause.