That or Which? That is the Question!

Many people have trouble deciding whether to use which or that when writing sentences such as the following:

1. The cars which/that have manual transmission are all on sale.
2. The math problems which/that demonstrate the Pythagorean Theory stumped the children.

Before we look at whether you should select which or that above, let's first talk about the function of the two. Although which and that can be used in other ways, in these instances they are acting as relative pronouns that are introducing adjective clauses. (Remember that a clause contains a subject and a verb but does not in itself express a complete thought.) The clauses are both modifying the nouns that precede them (cars and problems).

Now that we know what they do, let's look at how to choose the correct pronoun for the sentences. You must first determine whether or not the clause is necessary to the meaning of the sentence. Actually, in both sentences, you could use which or that. Your meaning holds the key. As Woe is I points out, the general rule of thumb is this:

"If you can drop the clause and not lose the point of the sentence, use which. If you can't, use that" (p. 3).

In other words, you have to decide whether the clause is restrictive (esential) or non-restrictive (non-essential).

Sentence One: If you mean that all the cars are on sale and that they all just happen to have manual transmission, then the clause is non-restrictive, and you should choose which. On the other hand, if you mean that you have other types of cars and only the ones with manual transmission are on sale, then the clause is restrictive, and you must choose that.

Sentence Two: If you mean that all the problems stumped the children and that they all just happened to have demonstrated the Pythagorean Theory, then the clause is non-restrictive, and you should choose which. If, however, you mean that there are other types of problems and that the only ones that stumped the children were the ones that demonstrated the Pythagorean Theory, then the clause is restrictive, and you must choose that.

Now for one more annoying point: If the clause is restrictive (using that), then you don't need any punctuation. If the clause is non-restrictive (using which), then you should separate it from the rest of the sentence with commas.

It's not that difficult, but it can be a bit confusing. Woe is I does a better job than I do of cutting to the meat of the matter with this little jingle:

"Commas, which cut out the fat,
Go with which, never with that" (p. 4).

This does take practice. Just remember first to determine whether or not the clause is vital to the meaning of the sentence. If it is, use that with no punctuation; if not, use which surrounded by commas.

If you feel like you still need some practice, visit at these online resources:

Get it Write
The Blue Book of Grammar and Punctuation