By Jacqueline Fawcett, RN; PhD; ScD (hon); FAAN; ANEF
Writer’s Camp Counselor
Copy editors often correct “which” and “that” and I decided to find out why.
Many years ago, I wondered why a copy editor for one of my books changed some sentences where I had used which and other sentences where I had used that. I followed the copy editor’s recommendations—a writer should always seriously consider the assistance of copy editors, who have “eagle eyes” for all things spelling, grammar, and consistency of citations and references—and finally discerned when to use which and when to use that. It had to do with when to place a comma before inserting which in a sentence and no need for a comma when inserting that.
However, a deeper dive revealed that it is much more complicated than just commas. I never fully realized, until I began to write this paper, the correct grammatical reason for their use. I consulted several websites and am happy to share my research with you.
What the Dictionaries Say
The Merriam-Webster Dictionary website1 has this guidance:
That and which can both introduce a restrictive clause, that is, a clause that can’t be removed without changing the sentence’s meaning substantially or making the sentence incomplete or difficult to understand:
The monitor that they bought was expensive.
The monitor which they bought was expensive.
In both examples, if you leave off the “was expensive,” the sentence doesn’t have a verb (was) or adjective (expensive), describing the monitor. Even though both sentences are technically correct, the first one, with that, sounds better.
Which is used to introduce a non-restrictive clause, that is, a clause that adds information but isn’t essential for understanding the sentence’s basic idea:
The monitor, which was very precise, was expensive.
The monitor, that was very precise, was expensive.
The monitor was expensive.<– The sentence makes sense without the “very precise” information. The first sentence is correct; the second is acceptable but doesn’t sound as good. A copy editor would probably edit this sentence to use which.
Elaborating, the website told me “if [a] clause is bracketed by commas . . . it is likely a non-restrictive clause, and you can give it a which. If it is not surrounded by commas, then it is most likely a restrictive clause, and you can choose to give it a that or a which.”
I tend to use which only when the clause is non-restrictive, that is, bracketed by commas. By doing it this way, you can remember the rule a little more easily (when you see commas, use which). Another way to remember: if the phrase is non-essential it is non-restrictive (non and non), therefore commas are needed and which is correct. In the example above, the first sentence is more technically correct and it sounds better. It is what we are used to hearing and reading.
The dictionary.com website2 had information that was slightly different from the Merriam-Webster dictionary:1
In formal American English [but not necessarily in British English or any type of informal English] . . . If the relative clause contains information that is not essential to the meaning of the sentence, and is also preceded by a comma, a dash, or parenthesis, it’s probably non-restrictive, so use which. If not, odds are it’s restrictive, so use that.2
The study data, which were drawn from an existing database, were verified and cleaned as the first step.
The study data that were part of a secondary analysis were verified and cleaned as the first step.
Notice the difference between the two—but they are both correct.
The Purdue Online Writing Lab: the OWL
The Purdue Owl website3 advice is similar to that from the Merriam Webster dictionary, with the addition of a few examples:
That versus Which
One of the more challenging grammar concepts in the English language is the difference between the words that and which. Both serve a similar purpose, but the meaning of the sentence can change depending on which one you use. The resources below break down the grammar rules associated with that and which and describes when to use each one.
Using That
Whether to use that and which depends on whether the clause it introduces is restrictive or non-restrictive. A restrictive clause means that the information in the clause is necessary to understand the preceding noun. For a restrictive clause, use that.
Let’s consider some more example sentences:
Mary’s stethoscope that has green tubing was a graduation gift from her parents.
In this sentence, it suggests that Mary owns multiple stethoscopes, so it’s important to distinguish the one with the green tubing from the others.
But if you write:
Mary’s stethoscope, which has green tubing, was a graduation gift from her parents.
Take out the “which has green tubing” and the sentence still makes sense. It also suggests that Mary only owns one stethoscope.
The laptops [plural noun] that are used for data collection purposes [restrictive clause] are usually more secure.
Here, because not all laptops are used for data collection, that is used indicate the necessary information. Also—no commas.
The chair [noun] at the nurses’ station that is missing a wheel [restrictive clause] is dangerous to sit on.
Once again, this sentence indicates there are multiple chairs at the nurses’ station, making it important to include a restrictive clause. Note that the restrictive clause does not necessarily need to be positioned immediately after the noun.
Using Which
Use which when the information in the clause isn’t necessary to understanding the noun in the sentence. This sort of clause is a non-restrictive clause. There is one other important distinction between restrictive and non-restrictive clauses: commas are used to separate the non-restrictive clauses from the rest of the sentence.
Stacy’s iPad [noun], which she uses for data collection [non-restrictive clause], has a cracked screen.
Here, the information about Stacy’s iPad being used for data collection is not necessary to the sentence. Stacy only has one iPad, so the extra information doesn’t help identify it. Therefore, which should be used as it is a non-restrictive clause and separated with commas.
Paul’s iPad, [noun], which he uses for data collection as well as taking notes in class, [non-restrictive clause], is in excellent condition.
Once again, the information about the data collection and note taking is not needed to understand the rest of the sentence. The non-restrictive clause could be omitted and the sentence would still make sense: Paul’s iPad is in excellent condition. We can also wonder about the care that Paul uses with his electronics compared to Stacy, but that’s a different story.
The human heart [noun], which contains four valves [non-restrictive clause], weighs approximately eleven ounces.
In this example, because all human hearts have four valves, the descriptive clause does not provide necessary identifying information in the sentence. Thus, “which” is used.
Parallelism
Just when you thought you had which and that figured out, a new wrinkle appears—the issue of parallelism.
“Sentence elements that are parallel in thought and function must be parallel in form.”4
“Parallel elements in a sentence are two or more words, phrases, or clauses that occur together and have the same grammatical function and the same importance of thought. Parallel structure, when used correctly, creates a symmetrical, graceful construction that is pleasing to the reader. Parallel structure used incorrectly can make writing appear sloppy and can confuse a reader.”4
Parallel structure can refer to verbs, adjectives, phrases, sentences, lists, and clauses. For now, I am only considering the special case of restrictive and non-restrictive clauses and the use of which and that.
The study data, which were drawn from an existing database, revealed that the participants’ experiences were positive.
Even though the commas suggest a non-restrictive clause (and it is), because parallelism is more important in this sentence, it is incorrectly written. It should be:
The study data that were drawn from an existing database revealed that the participants’ experiences were positive.
The first part could be omitted and the sentence would still make sense: The study data revealed that the participants’ experiences were positive. Overall, you could probably argue that the participants’ experiences are more important than where the study data came from.
Another example:
The nurses’ union, which had 1000 members, considered which contract to ratify.
The number of members is not necessarily essential (although useful information). However, “which contract to ratify” makes it clear that there is more than one contract being considered and that is important information.
Let’s end this section with a famous example, from the Gettysburg Address of President Abraham Lincoln—the concluding sentence:
That this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth.
Which, given the times we are in, is probably a good thing to remember.
People
When referring to people, do not use which or that, instead use who or whom.
The nursing staff, which were very thoughtful people, were planning a surprise party for their manager.
Instead:
The nursing staff, who were very thoughtful people, were planning a surprise party for their manager.
When In Doubt
Whenever you are in doubt about what word to use, consider rewriting the sentence to avoid the use of which and that altogether.
The manager’s surprise party was planned by her thoughtful staff.
The thoughtful staff planned the manager’s surprise party. <– Better, because the voice is more active.
The staff knew they needed to handle the expensive monitor with care.
The iPad with the cracked screen needed to be repaired before it could be used for data collection.
As a bonus, these sentences are more complex and thus more interesting to read.
Conclusion
To summarize:
- Which is generally used with non-restrictive clauses, while that is used with restrictive clauses.
- Non-restrictive clauses usually have commas to set them off, and are not required. Sentences still make sense without them, which can be a test for which word to use.
- Non-restrictive, not required, commas: which (no, no, co: which)
- Restrictive, required, no commas: that (re, re, no-co: that)
- In a sentence that uses that (or which) multiple times, be aware of parallel construction and use the same word.
- When referring to people, use who or whom, not which or that.
- When in doubt, consider rewriting the sentence.
References
- Merriam-Webster Dictionary. ‘That’ vs. ‘Which’ Which one should you choose? Accessed May 11, 2025. https://www.merriam-webster.com/grammar/when-to-use-that-and-which
- Dictionary.com. “That” vs. “Which”: When Do You Use Each?Accessed May 11, 2025. https://www.dictionary.com/e/that-vs-which/
- Purdue Owl. That vs. Which. Accessed May 19, 2025. https://owl.purdue.edu/owl/general_writing/grammar/that_vs_which.html
- Parallelism. Accessed May 19, 2025. https://www.hamilton.edu/academics/centers/writing/style/essentials/parallelism
© 2025 Writer’s Camp and Jacqueline Fawcett. CC-BY-ND 4.0
Author: Jacqueline Fawcett
Editor: Leslie H. Nicoll
Citation: Fawcett, J. The Grammar Camp Counselor: Using Which or That—That is the Question. The Writer’s Camp Journal, 1(1), 9. doi:10.5281/zenodo.15475319
Thanks for all this info on which vs. that! I get especially edgy about that vs. who and I like to say “things that” and “people who.”