Clarity: “That” and “which”

“That” and “which” are often mixed up in writing because the difference between them is extremely easy to miss. But both pronouns communicate something specific about the ideas contained in a sentence, and they are not, in fact, interchangeable.


“That” introduces a clause that provides essential information about the main idea of a sentence.

I want the dog that has spots

This sentence tells the reader that the spots are essential. The dog must have spots, no other dog will do.


“Which” introduces a clause that provides non-essential information and must be separated from the remainder of the sentence with a comma.

I want the dog, which has spots

This sentence tells the reader that the dog is the main idea, and that the dog in question only happens to have spots. The “which” clause could be dropped without losing the essential meaning: “I want the dog."

The “which” clause must always take a comma because it interrupts the main idea of a sentence.