By Jennifer Copley (Last Updated, November 11, 2013)
If it’s a clause that goes inside commas, use which; otherwise, use that:
- The cat, which is named Sockington, has 1,382,913 followers on Twitter.
- The cat that has 1,382,913 followers on Twitter is named Sockington.
In the first sentence, you could get rid of the clause (which is named Sockington) and you would still have a full sentence: The cat has 1,382,913 followers on Twitter.
In the second sentence, if you eliminate the portion containing that (The cat that has 1,382,913 followers on Twitter), you would be left with an incomplete sentence (Is named Sockington).
That indicates a particular one among many: The cat that has 1,382,913 followers on Twitter as opposed to all the other cats on Twitter that don’t have such impressive followings.
Which introduces information that can be considered extra; that precedes information that is essential to the meaning of the sentence. Therefore, if you have an important clause that needs to be set apart with commas (and the rest of the sentence makes sense on its own), use which. Otherwise, use that.
A which clause can also come at the end of a sentence:
She threw away the chicken carcass, which the dog retrieved from the garbage a few minutes later.
In this sentence, She threw away the chicken carcass stands on its own; the dog’s actions are separate. Using that changes the meaning of the sentence:
She threw away the chicken carcass that the dog had retrieved from the garbage.
In this case, she gets rid of the chicken carcass that the dog has been eating. The dog’s actions are essential to the meaning of the sentence because it doesn’t refer to any old chicken carcass; the carcass she threw away is the one that the dog had extracted from the garbage can and enjoyed until he was caught in the act and had his treasure taken from him.
For those who want to know the technical terms, which indicates a nonrestrictive clause because it supplies non-essential information (information not critical to the meaning of the sentence), whereas that introduces essential information and is therefore associated with restrictive clauses.