Even the best writers make mistakes when it comes to using “that” or “which” in a sentence. While both are relative pronouns, used to introduce clauses and help avoid overly choppy writing, they’re not interchangeable. (Well, unless you’re writing in British English.) So what’s the story with “that” and “which,” and when and how do you use them correctly?

That Versus Which

Just like adverbs and adjectives, relative pronouns modify words. Unlike adverbs and adjectives, however, they typically modify an entire clause, rather than a single word. Whether you use “that” or “which” depends on the nature of the clause you’re modifying.

Use “that” with restrictive clauses.

Restrictive clauses are necessary to help you understand the overall meaning of a sentence. We typically modify them with “that.” For example:

Books that are super-suspenseful are a lot of fun to read.

In our example, “that are super-suspenseful” is a restrictive clause. Remove it, and it’s unclear what types of books you’re referring to, which is essential for understanding the crux of the sentence.

Use “which” with nonrestrictive clauses.

Nonrestrictive clauses are ones that aren’t essential for you to understand the overall meaning of the sentence. You can remove them without really losing too much meaning. We typically modify them with “which.” For example:

Books, which you can increasingly find online, are a great tool for learning.

Removing the nonrestrictive clause “which you can increasingly find online” doesn’t change the fact that the writer is saying that books are great for learning. Rather, this nonrestrictive clause is simply adding extra information that we don’t technically need to grasp the intent of the sentence.

The Comma Trick

If grammar terms like restrictive clauses and relative pronouns make your eyes glaze over, you’re not alone. Here’s a nifty, alternative way to help you use “that” and “which” correctly. If you would naturally put a comma before and after the clause, then it’s a nonrestrictive clause, and you need to use “which.” If, however, you wouldn’t put commas around it, then it’s a restrictive clause and you should use “that.”

Exceptions to the Rule

There are, of course, exceptions to the rule. Many writers will interchange “that” and “which” if repetition becomes an issue. Take, for example, the classic “That which doesn’t kill you makes you stronger.” Technically, it’s incorrect. But, I think you’ll agree, “That that doesn’t kill you makes you stronger,” just sounds wrong.

Watch this space for more quick and dirty grammar tips.

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