The Rules for “That” and “Which” in Relative Clauses

A relative clause is a clause, with a subject and a verb, that describes a noun in another clause. Check out the phrase “that got away” in this line from the Civil Wars, a line that’s been stuck in my head all day:

“Oh, I wish you were the one that got away.”

“That” is both the subject and the relative pronoun in the relative clause, and “got away” is the verb. (It’s a phrasal verb, which is why it consists of two words.) The relative clause describes the word “one” and specifies to listeners which “one” they’re singing about.

“That” and “which” might be the two most common relative pronouns. Their similarities make it difficult to distinguish when to use them.

"That" and "which" have a lot in common.

In fact, I’ll start with the similarities. Both “that” and “which” are relative pronouns but can also be adjectives (“which one”) or pronouns (“That is the one.”). Both are typically used in relative clauses that describe objects. However, in less formal situations, either one could be used to describe a person, such as “the one that got away.” Remember that this works in less formal writing, not academic writing.

Use "that" without a comma.

Now for the differences. The relative pronoun “that” has a fairly strict set of rules, with few exceptions. 

  1. “That” is used only in essential clauses. In other words, any relative clause introduced with the word “that” must be absolutely necessary to the logic and meaning of the overall sentence.
  2. Never use a comma before “that.” Just don’t. No exceptions.
  3. Relative clauses that begin with “that” usually describe the noun that comes immediately before it. As an example, in the previous sentence, the relative clause “that comes immediately before it” describes the noun “noun.” There is one exception to this. Sometimes, you’re allowed to stick a prepositional phrase in between a noun and it’s “that” clause. Just make sure that it’s absolutely clear which noun the “that” clause refers to.
    • Not okay: “Please get the painting off the table that I stole.” What did you steal, the painting or the table?
    • Okay: “My favorite is the painting on the table that’s a landscape.” The sentence isn’t great, but it’s okay.


Use "which" with a comma, usually.

The relative pronoun “which” has a more confusing set of rules, but because of that, the word is a lot more forgiving.

  1. Use “which” mostly for nonessential clauses. In other words, use it when the clause could be deleted from the sentence without causing confusion.
  2. Sometimes you can use “which” for essential clauses, but only in informal writing. It also seems to be more okay to do this if you’re British. I don’t know why.
  3. Use a comma with “which” if you’re using it to introduce a nonessential clause. That means that you should always use a comma with “which” in academic writing, but no one will come find you if you don’t use a comma in other types of writing.
  4. Relative clauses that begin with “which” frequently describe the noun that comes before it, but not always. You can add a “which” clause onto the end of another clause and use the “which” clause to describe the entire clause. For example: “We’re almost out of coffee, which means I need to go to the store soon.


For this reason, “which” is a unique relative pronoun.

Using “which” and “that” incorrectly is a common mistake for native and non-native speakers. Using them confidently and correctly allows you to write more precisely and descriptively.

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