Pronouns help us avoid the cumbersome repetition of words. They stand in for nouns that we’ve introduced.
Imagine if we didn’t have the pronouns “you” and “me.” Instead of asking your friend Tom if he wants to go to the movies with you, you’d say: “Tom, does Tom want to go to the movies with Sue?” Sounds ridiculous, doesn’t it?
To avoid this, we use pronouns to take the place of nouns that we’ve already identified.
- Tom, would you like to go to the movies with me?
- The teacher told Bob he’d received the highest grade in the class.
- My parents said they were going out to dinner.
We do the same thing with the relative pronouns “which” and “that.” They are used to represent nouns that have already been identified, just as we did in the examples above. This being English grammar, of course, there are rules that govern their use. The most important of these is to determine whether they are part of a restrictive or nonrestrictive clause. (Note: Both words have other functions, as well: which as an adjective and that as a conjunction. In this article, however, we consider them only as pronouns.)
The Chicago Manual of Style defines a restrictive clause as one that defines a noun and/or adds specific details by providing information essential to understanding the sentence. A nonrestrictive clause explains or amplifies information about a noun and is usually set off from the rest of the sentence by commas. One way to identify a restrictive clause, in fact, is to see if you can remove it from the sentence without altering its meaning or rendering it incomplete.
Armed with this information, let’s take a look at how “which” and “that” are used.
“Which” is most often used in nonrestrictive clauses, usually separated by commas from the rest of the sentence, to provide further information about the preceding noun.
- The burning house, which until recently housed the Smith family, gave off a plume of thick, acrid smoke.
- As he pulled it from the fire, Kevin bemoaned the damage to his shirt, which he’d bought only that morning.
A restrictive clause can also be preceded by a parenthesis or dash.
- Hannah’s embarrassment (which was evident to all around her) left her red-faced and fuming.
- He stormed off in a huff—which was his usual way of dealing with conflict.
Note that in each of these examples, the restrictive clause (beginning with “which”) can be removed from the sentence without significantly changing its meaning or making it incomplete.
“Which” can be used in a restrictive clause if it’s preceded by a preposition.
- The house in which he was born was built in 1702.
- The war from which they fled took thousands of lives.
In these cases the restrictive clause is not bounded by commas, parentheses, or a dash.
“That” is is used in a restrictive clause to add specific descriptive elements to a preceding noun.
- Anyone that left the room was told they could not return until the following day.
- The boy that was hit by a car was airlifted to the hospital.
- After days of searching, the archaeologists found the artifacts that they were seeking.
Unlike nonrestrictive clauses containing which, you can’t remove the restrictive clauses (beginning with “that”) from the examples above without destroying the sentences. Notice too that restrictive clauses are not delimited with commas, parentheses, or a dash.
If you’d like to delve further into this subject, you could start with Merriam-Webster’s usage notes, which provide some interesting history on the evolution of the split between which and that. Grammarly (owned by Merriam-Webster) and Grammar Book also have helpful articles.
To learn about rules for other word pairs, try these TCK posts:
- Affect vs. Effect: Word Usage Explained
- i.e. vs. e.g.: Grammar Rules Explained
- Et Al. and Etc.—Whose Is Which?