Which vs That

Part of the reason I keep this blog is to practice writing. It's helped me identify nuances with the English language, which I would never have identified otherwise. One such nuance is the usage of which versus that and today I learned which to use when.

I Googled the topic and learned that it depends on whether the following clause is restrictive or non-restrictive. What does that mean? If the following clause only adds information to the sentence then use the word which. If the following clause is required for the sentence to make sense then use the word that. The Chicago Manual of Style has the following example:

"Pizza that's less than an inch deep just isn't Chicago style."

"Pizza, which is a favorite among Chicagoans, can be either bad for you or good, depending on how much of it you eat."

Note that if you remove "that's less than an inch deep" from the first sentence, it becomes inaccurate; i.e., it's not true that "pizza just isn't Chicago style." The clause, then, is restrictive (necessary) to the sentence; therefore "that" is correct. You could even leave "that" out:

"Pizza less than an inch deep just isn't Chicago style."

If, however, you take out the clause "which is a favorite among Chicagoans" from the second sentence, it still makes sense: i.e., pizza can be either bad for you or good, and whether or not it is a favorite among Chicagoans does not "restrict" this meaning; therefore the clause is nonrestrictive and should be introduced by "which" and set off by commas.

My mnemonic is that if the clause adds 'whatever, it's not important' to the sentence, then use what. Otherwise, use that. Let's see if I remember this.