That vs Which: What's the Difference?
Quick Answer
Use "that" for essential (restrictive) clauses without commas and "which" for nonessential (nonrestrictive) clauses set off by commas.
Definition of "That"
"That" is a relative pronoun and determiner used to introduce a clause that identifies or defines the noun it follows. In these cases the clause is essential to the meaning of the sentence—remove it and the sentence's meaning changes or becomes incomplete.
Etymology: "That" comes from Old English þæt, originally a demonstrative pronoun meaning "the one" or "that one." Over time it picked up its role as a relative connector in English.
Definition of "Which"
"Which" is a relative pronoun and interrogative determiner used for asking questions ("Which one?") and for introducing clauses that give extra information about a noun. When used as a relative pronoun, "which" typically introduces nonessential clauses set off by commas.
Etymology: "Which" traces back to Old English hwilc, meaning "of what kind." It retains a slightly more formal or clarifying flavor when used in relative clauses.
Key Differences
| Feature | That | Which |
|---|---|---|
| Basic meaning | Introduces essential (restrictive) information | Introduces nonessential (nonrestrictive) information or forms questions |
| Usage | Use when clause defines or limits the noun (no commas) | Use when clause adds extra detail (use commas) |
| Punctuation | No commas | Comma(s) usually required for nonrestrictive clauses |
| Part of speech | Relative pronoun / determiner | Relative pronoun / interrogative determiner |
| Regional notes | Preferred in American English for restrictive clauses | British English sometimes uses "which" for restrictive clauses, but U.S. style guides favor "that" |
Example Sentences
Using "That" (restrictive)
- The book that explains the process is on my desk. (Only the book with the explanation matters.)
- I want the shoes that are comfortable for running. (The specific comfortable shoes are the ones chosen.)
- Students that study regularly tend to do better on exams. (Refers only to students who study regularly.)
- Find the file that contains last year's budget. (Only the file with that content is relevant.)
Using "Which" (nonrestrictive)
- The book, which has a red cover, belongs to the library. (Extra info: the red cover is not essential.)
- My laptop, which I bought last year, is still fast. (Parenthetical detail about purchase.)
- This policy, which was updated in March, affects all employees. (The update is additional context.)
- We chose the blue paint, which matched the curtains perfectly. (Color detail is supplementary.)
Memory Trick
Mnemonic: "That’s Tight; Which is Wobbly." Think: "that" pinches the noun tightly—no commas allowed—because the clause is essential. "Which" waves a comma like a little flag of extra information.
Another quick tip: If you can remove the clause without changing the sentence's core meaning, use "which" with commas; if removing it changes meaning, use "that" without commas.
Quick Quiz
- I visited the museum ___ houses the ancient coins. (that / which)
- The museum, ___ houses the ancient coins, is closed on Mondays. (that / which)
- She prefers songs ___ rhyme with the chorus. (that / which)
- My car, ___ I washed yesterday, looks shiny. (that / which)
Answers
- that — the clause defines which museum
- which — the clause adds extra info (use commas)
- that — restrictive: she prefers only those songs
- which — nonrestrictive: washing is extra detail
Practical Tips You Can Use Right Now
- When in doubt, remove the clause. If the sentence still identifies the same thing, use "which" with commas; if it loses its meaning, use "that."
- Follow regional style: use "that" for restrictive clauses in American English; British English tolerates "which" more often.
- Proofread for commas—commas almost always mean "which." No commas usually mean "that."
If you write a lot, try rewriting a few sentences with both options to see how meaning changes. Tools like Rephrasely's paraphraser and AI writer can help generate alternatives quickly at https://rephrasely.com/.
Want to check whether your text was written by AI or needs humanizing? Use Rephrasely's AI detector. To ensure originality, try the plagiarism checker. And if you need help composing clear examples, the AI writer (Composer) can draft variations for practice.
Frequently Asked Questions
Can "which" ever be used without commas?
Yes—especially in British English, "which" sometimes introduces restrictive clauses without commas. However, American style guides usually prefer "that" for restrictive clauses to avoid ambiguity.
Is it always wrong to use "that" with commas?
Generally, yes: "that" rarely appears in nonrestrictive clauses set off by commas. If you have commas, "which" is normally the correct choice because the clause is parenthetical.
How can I practice choosing correctly?
Practice by editing sentences: remove the clause and check if the noun remains clearly identified. For bulk practice or rewriting suggestions, try Rephrasely's paraphraser and composer tools at https://rephrasely.com/ to generate alternatives and reinforce the rule.