“The cake that they served at the party was chocolate.” “The cake, which they served at the party, was chocolate.” Same words, one comma, two different meanings — the first tells you which cake, the second assumes you already know. That tiny punctuation mark is the entire secret of *that* versus *which*, a pair that trips up even fluent writers.
Both words introduce relative clauses and point at things, and plenty of published books use them interchangeably. But one reliable test — the comma test — settles nearly every case.
That vs Which: The Short Answer
Use *that* for restrictive clauses (essential information, no commas) and *which* for nonrestrictive clauses (extra information, set off by commas). A restrictive clause defines which thing you mean: “the car that won the race.” A nonrestrictive clause adds a bonus detail about an already-identified thing: “the car, which won the race, is red.”
One honest caveat, straight from Merriam-Webster: “You can use either *which* or *that* to introduce a restrictive clause — the grounds for your choice should be stylistic — and *which* to introduce a nonrestrictive clause.” In other words, restrictive *which* is a stylistic choice, not a crime — though in American editing, *that* is the safer default.
Restrictive vs Nonrestrictive: The Core Idea
A restrictive clause restricts the meaning — it tells the reader which one you mean. Remove it, and the sentence loses its target. “The report that landed on my desk this morning is urgent.” Which report? The one from this morning. Delete the clause and you’re pointing at nothing.
A nonrestrictive clause, by contrast, is a parenthetical bonus. “The report, which landed on my desk this morning, is urgent.” The report is already identified (presumably the only one under discussion); the clause just adds color. Remove it and the sentence still knows its target. This essential-versus-extra distinction is the engine of the whole rule — the commas are just the visible signal.
When to Use “That”
Use *that* when the clause is essential to identifying the noun — the restrictive case. “The password that unlocks the vault changes weekly.” “She returned the dress that didn’t fit.” In both sentences, the clause does the identifying work; without it, the sentence drifts.
In American English, *that* is the standard choice for restrictive clauses — editors change a restrictive *which* to *that* almost reflexively. This is convention, not law, but convention carries weight in formal writing. When in doubt, choose *that*: it will never be flagged.
When to Use “Which”
Use *which* when the clause is extra information about an already-identified noun — the nonrestrictive case. “My laptop, which I bought in 2021, finally died.” “Karachi, which sits on the Arabian Sea, is Pakistan’s largest city.” Notice the commas hugging the clause: they are the visible sign of a nonrestrictive clause, and they travel with *which* as a pair.
The reverse error — writing *that* with commas — is the genuine mistake to avoid. “The laptop, that I bought in 2021, died” jars in every register. If you see commas setting the clause apart, the word is *which*, full stop. That pairing is the one half of this rule that carries no stylistic wiggle room.
The Comma Test in Practice
The teachable rule in one sentence: read the clause aloud — natural pauses and commas mean *which*; no pauses means *that*.
Watch it work on the classic example family. “The cake that they served at the party was chocolate” — no pauses, the clause identifies which cake, so *that*. “The cake, which they served at the party, was chocolate” — paused, comma-wrapped, the cake already identified, so *which*. Same content, different grammar, different meaning: the first answers “which cake?”, the second assumes the cake and adds a detail.
One dialect wrinkle: British writers use restrictive *which* far more freely, and many British style guides don’t teach the American *that*-only convention at all — in a British book, that *which* isn’t an error.
Common Mistakes
First, treating “never use *which* restrictively” as absolute law. It’s the American convention, not a grammatical ban — Merriam-Webster explicitly allows restrictive *which* as a stylistic choice.
Second, the real error: using *that* in a nonrestrictive clause. “The proposal, that won approval, was mine” is wrong in every register of English. Commas mean *which*; no exceptions, no stylistic wiggle room.
Third, punctuating *which* inconsistently. A nonrestrictive *which* clause needs both commas — opening and closing. Drop one and the sentence breaks; commas travel in pairs around these clauses.
Fourth, swapping in *who* for things. People get *who*; things get *that* or *which*. “The manager that approved it” is fine, “the manager which approved it” is not. For the people side of this whole family, see our guide to who or whom. And for another precision pair where the real rule beats the classroom myth, read between vs among.
Frequently Asked Questions
If the clause is set off by commas (nonrestrictive, extra information), use *which*. If it flows without commas because the clause identifies the noun (restrictive, essential information), use *that*. Commas and *which* travel together; *that* never takes comma wrapping.
No — it’s a stylistic choice, not a grammatical error, and Merriam-Webster says so explicitly. American editors conventionally prefer *that* there, and it’s the safer default in formal writing, but restrictive *which* is standard in British English.
A clause essential to identifying the noun it modifies. “The key that opens the garage is missing” — without the clause, we don’t know which key. Restrictive clauses take no commas and, in American convention, the word *that*.
A clause that adds extra information about an already-identified noun. “The key, which opens the garage, is missing” — we already know the key; the clause is a bonus. Nonrestrictive clauses are comma-wrapped and always use *which*.
The first is restrictive: the clause identifies which cake, implying there were others. The second is nonrestrictive: the cake is already known (the only one), and the clause adds background. The comma — not the word choice alone — carries the meaning shift.
Yes, in restrictive clauses — “the book which I borrowed” is a stylistic choice rather than an error, and it’s the norm in much British writing. But the conventional American choice there is *that*, and pairing *which* with restrictive meaning still raises eyebrows in American editing.
