Relative Pronouns: “Which,” “That,” and “Who/Whom”

You should be glad to know that the most important thing about using these pronouns is also the easiest to understand. In fact, you probably are entirely aware of it already, for it’s sim¬ply a rule that “which” can only be used to refer to things (whether they’re tangible such as beds or buildings, or intan¬gible such as ideas), but not to people. You’d never say, The plumber which you met yesterday goes to our church, al¬though you certainly could say, The plumber goes to our church, which is on the next block.” Because the plumber is a person, the clause describing him needs the pronoun in “the plumber that you met...” or “the plumber whom you met….”

There’s also a second, trickier rule involving the difference between “which” and “that.” To be honest, you’re not likely to get into much trouble by neglecting this rule; it’s sometimes disregarded by people whose English is otherwise very good, but it is a rule that makes sense and that’s observed by most careful speakers and writers. Explaining it calls for a bit of specialist’s language, but examples should make things pretty clear.

Accordingly, let’s consider two sentences. Each of them uses the relative pronoun correctly, and we’ll shortly under¬stand why this is the case.

The cat, which had been sleeping for hours, woke up when the canary sang.

The cat that had been sleeping for hours was hungrier than the cat that ate the canary.

Now, in the first sentence, the clause “which had been sleeping for hours” gives us some information about the cat, but it isn’t essential to the meaning of the sentence; we know the significant fact, that the cat awoke, whether or not we have the additional information contained in the clause, “which had been sleeping.” This clause is, we can say, nonessential, or, as the grammarians put it, a nonrestrictive clause—nonrestric¬tive because, although it does describe what the cat has been doing, it doesn’t restrict or limit the meaning of the principal clause; the cat awoke, regardless of how long it had been sleeping. Because our clause is thus nonrestrictive, or non¬essential, we indicate its “expendable” nature by using the pronoun “which” and, in writing, by setting off the clause with commas.

In the second sentence, on the other hand, the clauses be¬ginning with “that” are clearly essential to the meaning of the sentence, which would otherwise only tell us that one (unspeci¬fied) cat was hungrier than another (unspecified) cat. For the sentence to do any sort of job, it must narrow its meaning down; it must distinguish between cats, between the sleeper and the canary-eater, and thus restrict the principal action to two par¬ticular, clearly different creatures. These restrictive clauses are not expendable; they are essential to and inseparable from the basic meaning of the sentence. They are signaled by the pronoun “that,” and, in writing, they are not set off by commas.
As you can see, the rules can be pretty clearly stated:
Use which in a nonrestrictive clause (a clause not essential to the meaning of the sentence).
Use that in a restrictive clause (a clause essential to the meaning of the sentence).

There’s one pretty obvious exception to these rules. If you have a nonrestrictive clause (calling for “which”) but you are referring to a person, follow the earlier rule (and probably your own instinct); avoid the “which” and go back to “who” or “whom.” So it’s proper (and certainly natural) to say:
Bob, who (and not, in this case, “which”) had been spending his day fishing, ate all the hot dogs.

With a restrictive clause referring to a person, you can, as with all restrictive clauses, use “that,” or, as many people pre¬fer, “who” or “whom.” Here are a few examples:

The man that you just insulted is my brother-in-law.

Or: The man whom you just insulted is my brother-in-law.


Which vs. That


Don’t Say: The invitations which we sent by carrier pigeon arrived sooner than the invitations which we sent in the mail.
Say Instead: The invitations that we sent by carrier pigeon arrived sooner than the invitations that we sent in the mail.

Here’s Why: In this case “that” is correct, because the clauses are restrictive: They help us distinguish one group of invitations from another group. Here are additional correct ex¬amples featuring “which” and “that”:

Is she the one that you broke up with?

The tennis ball that the dog played with was wet.

Sharon is the one that I’m going to marry.

The car, which had been stolen only an hour before, was found stripped down to its frame.