If you're confused about that vs. which, don't feel bad. It's one of the most common topics people ask me about, and it all boils down to restrictive clauses.
If you're confused about that versus which, don't feel bad. It's one of the most common topics people ask me about. I used to work as a technical writer, and I'd often edit documents in which people used the wrong word. More than once, I'd put in the right word, only to have clients change a perfectly fine that to a which and send it back to me. In fact, having a client try to overrule my correction of a which to a that was one of the things that pushed me over the edge and made me start the Grammar Girl podcast.
That vs. Which: What's the Difference?
Here's an easy way to remember the difference between that and which: If removing the words that follow would change the meaning of the sentence, use "that." Otherwise, "which" is fine. Some people will argue that the rules are more complex and flexible than this, but I like to make things as simple as possible, so I say that you use that before a restrictive clause and which before everything else.
A restrictive clause is just part of a sentence that you can't get rid of because it specifically restricts some other part of the sentence. Here's an example:
- Gems that sparkle often elicit forgiveness.
The words that sparkle restrict the kind of gems you're talking about. Without them, the meaning of the sentence would change. Without them, you'd be saying that all gems elicit forgiveness, not just the gems that sparkle. (And note that you don't need commas around the words that sparkle.)
A nonrestrictive clause is something that can be left off without changing the meaning of the sentence. You can think of a nonrestrictive clause as simply additional information. Here's an example:
- Diamonds, which are expensive, often elicit forgiveness.
Alas, in Grammar Girl's world, diamonds are always expensive, so leaving out the words which are expensive doesn't change the meaning of the sentence. (Also note that the phrase is surrounded by commas. Nonrestrictive clauses are usually surrounded by, or preceded by, commas.) Here's another example:
There was an earthquake in China, which is bad news.