Do you ever wonder whether you should use “which” instead of “that” or vice versa? In the United States we use “which” differently than those living in the United Kingdom. I have no idea when or how the usage diverged, but according to our Modern Language Association, there is a correct way to use each.

In Patricia T. O’Conner’s brilliant book Woe Is I, she addresses this issue in a segment she calls “The Which Trials”—a very fun read. Using her approach, let me ask you this: Which one of these two sentences sounds better to you?

Everyone loves a day that is sunny.

Everyone loves a day which is sunny.

What we’re looking at here is the choice between these two clauses: “a day that is sunny” and “a day which is sunny.” A clause is a group of words that has its own subject and verb.

The correct choice is the first one shown: “Everyone loves a day that is sunny.”

There are two criteria to help you figure out the correct usage:

1. “If you can drop the clause and not lose the point of the sentence, use which. If you can’t drop it, use that” (O’Connor 3).

2. “A which clause goes inside commas. A that clause doesn’t” (O’Connor 3).

Here are two examples that illustrate these concepts.

1. Saturday, which was the sunniest day of the week, felt like Spring.

2. The day that was the sunniest day of the week felt like Spring.

The point of both sentences is that there was a day this last week that was the sunniest day of the week, and it felt like Spring on that day.

Now let’s see what happens when we remove the that or which clause. In #1 we can remove the which clause, and the core meaning of the sentence remains intact: Saturday felt like Spring.

In #2, if we remove the that clause, we’re left with “The day felt like Spring.” We now have an incomplete message and you would naturally ask, “Well, which day was that?” The answer would be, “The day that was the sunniest day of the week.” Ah, so now we might remember that Saturday was the sunniest day of last week, and we could agree that yes, it did feel like Spring.

Many Americans think that the British way of speaking is more elegant and would choose which over that in order to sound more sophisticated. And while this may be true, it is NOT correct American English. “In fact,” according to O’Connor, “that is more likely to be grammatically correct than which” (4). She explains that this is so because most of us “don’t put unessential information in the middle of our sentences, especially when speaking”(4).

I like her little memory aid, which she titles “Comma Sense”:

Commas, which cut out the fat,

Go with which, never with that” (4).

If you choose to use a which clause, you will want to enclose it in commas—because it is not crucial to the meaning of the sentence and could be removed. It would be considered an aside that adds some interest to the sentence but could be left out. If you choose to use a that clause, you will NOT want to enclose it in commas because it will be essential to the meaning of the sentence.

I hope you enjoyed today’s grammar lesson. See you next time!

Source: O’Connor, Patricia. Woe Is I. New York: G. P. Putnam’s Sons, 1996. Print.

Susan is a retired English professor of 25 years who enjoys all that her home state of Colorado offers. She is an avid road bicyclist, hiker, and viola player with six published books who has really, really—yes really—enjoyed teaching grammar and mechanics to her students.