Which Versus That

Here's the deal: 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.

Restrictive Clause—That

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.)

Nonrestrictive Clause—Which

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.

Here, leaving out the words 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.

Credits: Grammar Girl