The correct use of that and which can be tricky. It’s a grammar question that has to do with restrictive clauses, a type of dependent clause.
Whether you choose that or which depends on the situation—in the U.S., at least. Both that and which are relative pronouns (as described here) that introduce a dependent clause.
Dependent clauses come in two flavors: restrictive and nonrestrictive.
Restrictive Clauses Contain Key Information
Before we talk about the specific kind of dependent clause called a restrictive clause, we should define the former so we can make sense of the latter.
What is a dependent clause? Well, its name is a good hint. It is dependent on another clause (an independent clause). In other words, it won’t appear by itself. Here’s a definition:
dependent clause: a part of a sentence that has a subject and a predicate but that cannot stand alone
For example, each sentence below contains a dependent clause (in bold):
- The girl had a dragon tattoo that frightened her employer.
- The girl wore a pearl earring, which the artist used as a focal point.
So what’s the difference? The clause in Sentence 1 is grammatically restrictive. All this means is that it contains key information.
In the example, dragon tattoo has more meaning thanks to the addition of that frightened her employer. Take it away and the meaning of the sentence changes drastically: The girl had a certain type of tattoo.
The structure of the sentence gives more thematic weight to dragon tattoo than to other elements in the sentence. The word that begins restrictive clauses, in this case one that tells us a lot about the power that this tattoo had.
In Sentence 2, however, the clause starting with which is grammatically nonrestrictive. Nonrestrictive clauses are not essential to the sentence as a whole.
Put another way, you could remove the which-clause and the crux of the sentence would still be the same (information about a girl’s choice of jewelry).
Heck, the information in the which-clause is almost an afterthought.
Notice the comma before the which-clause (nonrestrictive). There is no comma before a that-clause (restrictive).
Restrictive Clauses Take that, Not which
So how do you decide whether you need that or which? Just ask yourself the following question: Am I presenting essential information? If so, use that. If not, use which.
Here’s another pair of example sentences:
- The app that Chris downloaded yesterday doesn’t work.
- The organization, which was founded in 1934, provides funding to needy families.
In Sentence 1, the clause contains essential information: the app in question is the one that Chris downloaded yesterday. (Presumably, all other apps that Chris has downloaded work.)
In Sentence 2, though, the dependent clause contains nonessential information. We don’t need to know when the organization was founded to know what its mission is. In fact, we can take away the clause that begins with which and we still know what the organization does.
More Examples of Restrictive Clauses
Here are a few examples that should help you understand restrictive clauses better. In each sentence, the information contained in the that-clause is essential to the meaning of the sentence.
- The book that Jill borrowed is overdue.
- Mr. Jenkins bought a car that has no radio.
- The green pen that I lost on Tuesday was my favorite.
Now, if you’re British, the correct use of that and which is murkier. To my knowledge, British English doesn’t make a distinction between that and which.
The following sentence, which was taken from a recent Economist article, uses which for a restrictive clause and that for a second (embedded) restrictive clause:
Local dealers then cut it with other chemicals such as benzocaine, a local anaesthetic which simulates the numbing effect that real cocaine has on the gums.
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