Luckily, if all you’re interested in is communicating clearly in your written work, there’s a simple way of knowing which term is correct in any given situation.
The important thing to know is “that” and “which” are both pronouns used when introducing a relative clause (i.e., additional detail about the thing being described). However, each is used in a slightly different situation.
That (Restrictive Relative Clauses)
The term “that” is used when introducing a restrictive relative clause (i.e., a relative clause that restricts the meaning of the sentence):
All the houses that we rented were infested.
In the above, the phrase “that we rented” identifies the specific houses being described (i.e., the houses that were rented, not those we didn’t rent). It is thus a “restrictive” clause and cannot be removed without changing the meaning of the sentence.
The “which” clause here introduces new information, but isn’t essential to the sentence: It tells us that we rented “all the houses,” but not which houses we’re referring to.
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Here we see the difference between “that” and “which.” Since we could remove “which we rented” without changing the overall meaning of the sentence, it is a non-restrictive clause.
You’ll notice too that the “which” clause is contained within commas, indicating the use of a non-restrictive relative clause. We also use a comma when a non-restrictive relative clause appears at the end of the sentence:
I spent years living in infested buildings, which wasn’t much fun.
That or Which?
Keeping in mind these differences, knowing whether to use “that” or “which” at any given point simply requires asking, “Does this clause change the meaning of my sentence?”
If removing the additional detail would change the meaning of the sentence, you need to use “that.”
If removing the detail doesn’t change the overall meaning, you should use “which” and set the clause within parenthetical commas. Remember:
Restrictive relative clause (i.e., one that changes the meaning of the sentence) = That
Non-restrictive relative clause (i.e., one that doesn’t change the meaning) = Which