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The Basic Rule
The distinction between that and which is important in academic writing in the US. It may be less important in informal contexts and in other regions.
Use the word that to provide an essential explanation of the words that precede it. No comma is used before that. This choice makes sense if the reader will not understand what you are talking about without the that clause.
Use the word which to provide nonessential, relevant information related to the main point. Use a comma before the word which. This choice is appropriate if the sentence’s main point would remain unaltered if you delete the which clause.
A Simple Example
Imagine you are in a pet shop and there are 10 dogs. Your friend says to you, “I like the dog.” Of course, you have no idea which dog your friend is referring to. In this case, your friend could say “I like the dog that has a spot on its eye.” In this case, the word that provides an essential explanation of “the dog” so that the listener can understand the sentence.
If there is only one dog in the room, the that clause is unnecessary. In that case, you could say “I like the dog.” and the meaning would be clear. If you wanted, you could add some extra information with a which clause. For instance, “I like the dog, which [by the way] has a spot on its eye.”
A Helpful Guideline
Notice that I added the phrase by the way in that sentence. When you are trying to decide whether to use the word that or the word which in your sentences, a helpful guidelines is to use which if the sentence would still make sense if you added the phrase by the way. That is, if the information after this point is extra and not really fundamental to understanding the words that precede it, use which.
An Example in a Scholarly Context
If you are not careful, you can confuse your readers, or, worse yet, you can give an incorrect meaning about your methodology, results, and so forth. Take the following hypothetical example of a sentence in a Method section of a paper:
- The patients did not take any medications that lowered their metabolism.
- The patients did not take any medications, which lowered their metabolism.
Sentence 1 means that the study patients did not take medications that lowered their metabolism, but the sentence leaves the possibility open that the patients may have been taking other drugs (antidepressants, pain killers, or other medications that could have affected the results).
If the study design did not control for other medications, then of course Sentence 1 would be appropriate and the author would just have to note this as a limitation in the paper. However, if the author did not mean to leave open a possibility such as this, the incorrect use of the word that would indicate a flaw in the design that does not really exist.
Sentence 2 means that the study participants did not take any medications whatsoever (not to lower metabolism, fight depression, kill pain, or anything else). This is very different from Sentence 1. Additionally, Sentence 2 says that it was the very fact that the patients did not take any medications that lowered their metabolism.
That is, Sentence 1 seems to refer to a comparison between patients who took medication to lower metabolism and those who did not, whereas Sentence 2 seems to refer to a comparison between patients who took no medication versus patients who took any kind of medication. These sentences would make sense in different study designs; simply put, they are not interchangeable.
Why This Is Important
- Incorrectly describing your methodology makes it more difficult for other researchers to replicate or expand upon your research.
- Incorrectly describing your results or implications could ultimately affect public policy, programs, and the lives of many people.
- Incorrectly describing your theoretical framework will confuse your readers and make it more difficult for them to follow your analysis and interpret your results.
- Confusing your peer reviewers will result in a more negative overall perception of your paper.
- Writing clearly and precisely improves your reputation as a writer and researcher in the field, increasing the likelihood of collaborations with respected researchers in the future.