Quick answer: In traditional American English style, that commonly introduces a restrictive relative clause that identifies or limits a noun, while which commonly introduces a nonrestrictive clause that adds extra information. Restrictive clauses normally do not use commas. Nonrestrictive clauses normally use commas.
That vs Which at a Glance
| Word | Typical use | Punctuation | Example |
|---|---|---|---|
| that | Restrictive information | Usually no commas | The book that I bought is new. |
| which | Nonrestrictive information | Usually commas | My book, which I bought yesterday, is new. |
The key issue is not simply choosing one word. It is deciding whether the relative clause is essential to identifying the noun or merely adds information.
What Is a Relative Clause?
A relative clause is a group of words that gives information about a noun. It often begins with a relative word such as that, which, or who.
- The laptop that I bought yesterday is fast.
- The laptop, which I bought yesterday, is fast.
The words inside the clause help readers understand the noun. The punctuation and choice of relative word can change the relationship between the information and the noun.
When to Use That
Use that for a restrictive clause when the information is needed to identify which noun you mean.
Consider: “The students that studied passed the exam.” The clause “that studied” identifies the group of students being discussed. The sentence does not necessarily mean every student studied.
Another example is “The phone that has the cracked screen is mine.” The relative clause helps identify the specific phone.
When to Use Which
Use which for a nonrestrictive clause when the information is additional and the noun is already clear.
For example: “My phone, which I bought last year, still works.” The phrase “which I bought last year” adds information about a phone that is already identified as mine.
Nonrestrictive clauses are normally separated from the rest of the sentence with commas.
The Comma Rule
Commas are a major part of the traditional American English distinction.
| Meaning | Example |
|---|---|
| Restrictive | The cars that are electric qualify for the rebate. |
| Nonrestrictive | The cars, which are electric, qualify for the rebate. |
These sentences can communicate different ideas. In the first, the clause identifies a subset of cars. In the second, the sentence presents the cars as already identified and adds the information that they are electric.
The Removal Test
A useful test is to remove the relative clause. If removing it changes which noun you mean, the information is likely restrictive.
“The recipe that uses fresh basil won the contest.” Remove the clause and you get “The recipe won the contest.” The original clause helps identify which recipe.
Now consider “The recipe, which uses fresh basil, won the contest.” The recipe is already identified. The basil detail is additional information.
Can Which Be Used Without a Comma?
This is where grammar advice can become more nuanced. In traditional American editorial style, which is commonly associated with nonrestrictive clauses and commas. British English and some other varieties allow restrictive which more freely. Modern usage guides also recognize that real English usage is more varied than a single classroom rule.
For an American-focused grammar site, using that for restrictive clauses and which for nonrestrictive clauses is a clear and useful convention, especially in formal writing. The goal is clarity and consistency, not pretending that every variety of English follows exactly the same style rule.
That vs Which for People and Things
In standard usage, who is normally preferred when the antecedent is a person. That and which are more commonly discussed with things and animals, depending on context and style.
- The employee who called me left a message.
- The report that I read was useful.
- The report, which I read yesterday, was useful.
This helps prevent another common mistake: forcing that or which into a sentence where who is the natural choice for a person.
Common Mistakes With That and Which
| Common mistake | Better form | Reason |
|---|---|---|
| The book, that I bought, is new. | The book that I bought is new. | Restrictive that clauses are not set off by commas. |
| My car that is blue is new. | My car, which is blue, is new. | If the color detail is extra, use a nonrestrictive clause. |
| The person which called me… | The person who called me… | Who is normally used for people. |
That vs Which in Formal Writing
For academic, business, and professional writing, consistency matters. If the clause is essential to identifying the noun, write it without commas and use that when it fits the sentence. If the clause is extra information, set it off with commas and use which.
This approach is especially helpful when a sentence contains several nouns. Clear punctuation makes it easier for readers to see which information is essential and which information is additional.
Examples You Can Copy as Models
- The files that contain errors need review.
- The files, which contain confidential information, are stored securely.
- The restaurant that serves breakfast opens early.
- The restaurant, which opened last year, is already popular.
- The book that you recommended was helpful.
- My copy of the book, which you signed, is on my desk.
Practice: That or Which?
- The jacket ___ I bought yesterday is blue.
- My jacket, ___ I bought yesterday, is blue.
- The computer ___ has the new software is faster.
- Our computer, ___ is only two months old, is already slow.
Answers: 1. that 2. which 3. that 4. which. The exact choice can depend on the intended meaning, so first decide whether the clause identifies the noun or adds extra information.
Why the Difference Matters
The that-versus-which distinction can change the scope of a sentence. Consider “The reports that contain errors need revision.” This identifies a particular group of reports. Now compare “The reports, which contain errors, need revision.” The second version presents the reports as already identified and adds the claim that they contain errors.
This is why commas are not merely visual decoration. In a restrictive clause, the information helps identify the noun. In a nonrestrictive clause, the information is presented as an additional comment. A reader may therefore interpret the two versions differently.
A Practical Editing Checklist
- Find the noun being described.
- Ask whether the clause is necessary to identify that noun.
- If it is necessary, avoid commas around the clause and use the restrictive pattern.
- If it is extra information, use commas and the nonrestrictive pattern.
- Use who when a person is the natural antecedent.
- Check the style guide if you are writing for a specific publication.
Frequently Asked Questions
What is the easiest way to remember that vs which?
Ask whether the clause is needed to identify the noun. If it is, use the restrictive pattern, commonly that without commas. If it is extra information, use which with commas.
Does which always need a comma?
No. The traditional American rule strongly associates nonrestrictive which with commas, but English usage varies by variety and context.
Can that introduce a nonrestrictive clause?
In standard formal American writing, that is generally not used to introduce a nonrestrictive clause.
Should I use who for people?
Yes, who is normally the natural relative pronoun for people, especially in formal writing.
Related Grammar Guides
Improve your writing with our punctuation guides, sentence structure guides, your vs you’re, each vs every, and bring vs take articles.
Trusted Grammar References
For further reading, see GrammarBook on relative pronouns, Cambridge Dictionary on relative clauses, Merriam-Webster on that, Merriam-Webster on which, and Oxford Learner’s Dictionaries on relative clauses.
Conclusion
The practical American English rule is to use that for restrictive information and which for nonrestrictive information, with commas around the nonrestrictive clause. Because English usage varies across regions and styles, treat this as a useful convention rather than an absolute rule for every variety of English.