The difference between which and that is usually about whether the words that follow identify the noun or simply add extra detail. In standard US academic writing, use that for essential information and which with commas for nonessential information. The rule is simple. The difference between which and that becomes clearer when you ask whether removing the clause changes the basic meaning of the sentence.
Difference between which and that in restrictive clauses
A restrictive clause gives information you need to know exactly which person, object, idea, or group the writer means. Use that for this kind of clause in most US school essays. No commas appear. If you remove the clause, the reader may not know which noun you mean, or the sentence may make a different claim.
Consider this sentence: The sources that include publication dates are easier to evaluate. The clause that include publication dates identifies a particular set of sources. It does not mean every source is easier to evaluate. It means only the sources with dates are easier to evaluate.
Now remove the clause: The sources are easier to evaluate. That is a much broader statement. The meaning changed. This is a useful test, although it does not work perfectly in every sentence because context can supply some of the missing information.
Here is another example: Students that revise their thesis statements often write more focused drafts. The sentence refers to students who revise their thesis statements, not necessarily all students. The clause narrows the group. In formal US prose, that is the usual choice.
When to use which with commas
A nonrestrictive clause adds information that is interesting or useful but not necessary to identify the noun. Use which and place commas around the clause. The commas matter. They tell readers that the sentence would still point to the same noun if the extra detail disappeared.
For example: My literature review, which includes twelve peer-reviewed articles, needs a clearer conclusion. The writer has already identified one literature review. The detail about twelve articles adds context. Remove it, and the core meaning remains: My literature review needs a clearer conclusion.
Compare that sentence with this one: The literature review that includes twelve peer-reviewed articles needs a clearer conclusion. This version suggests there is more than one literature review. The clause tells readers which one needs work. Small change. Different meaning.
Try reading the sentence aloud with a brief pause where the commas appear. A nonrestrictive which clause often sounds like an aside. This reading trick is not a grammar rule by itself, but it can help you notice when you have added detail rather than identified the noun.
The grammar rule explained with one question
Ask yourself: Do readers need this clause to know exactly what I mean? If the answer is yes, use that without commas in standard US academic writing. If the answer is no, use which with commas. Start there.
- Find the noun that comes before which or that.
- Read the sentence without the clause. Notice whether the noun is still clear.
- Use that if the clause selects a specific noun or group.
- Use which with commas if the clause merely adds information about an already identified noun.
- Read the revised sentence once more. Check that the punctuation matches your intended meaning.
Suppose you write: The claim which appears in the final paragraph needs evidence. Ask whether your paper has several claims. If it does, the final-paragraph clause identifies one claim, so write: The claim that appears in the final paragraph needs evidence.
Now suppose your reader already knows you mean one specific claim: The central claim, which appears in the final paragraph, needs evidence. In that version, which appears in the final paragraph supplies extra location information. The commas show that the writer considers the claim already identified.
Why commas change the sentence
Commas are not decoration here. They control the meaning. A restrictive clause normally has no commas because it is tightly connected to the noun it defines.
Look at these two examples: Applicants who submit portfolios will receive feedback. This means feedback goes to the applicants who submit portfolios. Other applicants may not receive it. The clause limits the group.
Now add commas: Applicants, who submit portfolios, will receive feedback. This implies that all applicants submit portfolios, and the portfolio detail is extra. That may be an unintended claim. Be careful.
The same logic applies to which. Write The laptop, which has a cracked screen, needs repair when there is one already known laptop and the crack is added detail. Write The laptop that has a cracked screen needs repair when several laptops exist and you mean one particular laptop.
Common mistakes in student writing
One common mistake is using which without commas for a restrictive clause in a US academic essay. You may see that pattern in published writing, especially writing shaped by British conventions, but many US teachers and editors prefer that in this situation. Follow your assigned style guide. If no guide is assigned, using that for restrictive clauses is usually the clearest choice.
Another mistake is putting commas around information that actually identifies the noun. Consider: The evidence, that supports my argument, comes from the survey. This is incorrect in standard US usage because that does not normally introduce a nonrestrictive clause. The sentence also leaves the meaning uncertain.
You could write The evidence that supports my argument comes from the survey if you mean only some evidence supports the argument. Or write The evidence, which supports my argument, comes from the survey if all the evidence already under discussion supports it. Context decides.
A third mistake is assuming that which always means extra information. It often does when it follows a noun and commas, but which has other jobs. For instance, in Which source is most recent?, it begins a question. You cannot replace it with that.
Writers also use which after prepositions: The policy under which the class operates is outdated. Formal phrasing can sound stiff. In many student essays, a clearer revision is The policy that governs the class is outdated. Choose clarity over formality for its own sake.
American and British usage
The restrictive-that and nonrestrictive-which pattern is the safest default for US academic writing. British English more often accepts which in restrictive clauses without commas. That is a real convention difference. It is not automatically an error when you encounter it in a British publication.
Your audience matters. A paper for a US high school, college course, scholarship application, or graduate application will usually read more smoothly when restrictive clauses use that. A professor may have a different preference. Check the course instructions or the style guide named in the assignment.
Do not turn this into a mechanical replacement exercise. Grammar tools sometimes flag every restrictive which, but software cannot always tell what you intended to identify. You still need to decide whether the clause is essential. Meaning comes first.
Quick editing practice
Use this short practice set before you submit a paper. Each pair changes based on what the writer means. Read both versions carefully.
- The articles that were published after 2020 address the topic directly. Only articles published after 2020 are meant.
- The articles, which were published after 2020, address the topic directly. All of the already identified articles were published after 2020.
- The professor that teaches rhetoric assigned the essay. The clause identifies one professor among several.
- Professor Lee, which is not correct for a person, assigned the essay. Use who for people: Professor Lee, who teaches rhetoric, assigned the essay.
That final example points to a related rule. Use who or whom for people, depending on the sentence role, rather than which. In ordinary prose, that can refer to people in restrictive clauses, but who often sounds more natural: The student who asked the question stayed after class.
For an efficient final check, search your draft for the word which. Look at every case where it follows a noun. If there are no commas, ask whether that would make the restrictive meaning clearer. Then search for that after commas, because that combination often signals a punctuation or wording problem.
FAQ
Can I always use that instead of which?
No. Use which for nonrestrictive clauses, usually with commas, and for questions such as Which topic should I choose? You also need which after certain prepositions in formal constructions. Context matters.
Is which without commas always wrong?
No, but it may conflict with the convention expected in a US academic paper. British English often permits restrictive which without commas. If your teacher expects a US style, use that for restrictive clauses unless the assignment says otherwise.
Can that have commas around it?
Usually no when that introduces a relative clause. Write The book that changed my view is on the desk, without commas, when the clause identifies the book. If the information is extra, rewrite with which and commas.
How do I know whether information is essential?
Remove the clause and look at the result. If the reader can no longer tell which noun you mean, the information is essential. If the noun remains clearly identified and the clause only adds a fact, use which with commas.
Should I follow a grammar checker’s suggestion?
Use the suggestion as a prompt to review the sentence, not as a final decision. A checker can spot a pattern. It cannot reliably know whether you mean one specific noun or a whole category without enough context.
The practical rule is straightforward: use that without commas when the clause identifies the noun, and use which with commas when the clause adds nonessential detail. Keep your meaning in view. Then let punctuation show readers how to read the sentence.
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This article is for general informational purposes only and is not academic, admissions, or legal advice. Tool features, detection accuracy, and academic integrity policies change, so always verify current guidelines with your school or the official tool provider before making a decision.