That vs which: the rule American editors follow, a quick test, and when a comma changes the meaning
Use "that" when the clause is essential to identify what you mean, with no commas. Use "which" when the clause just adds extra information, set off by commas. That is the rule most American book editors follow. Here is the two-second test, examples where getting it wrong changes the meaning, and what a copy editor will do with your manuscript.
The rule
The Chicago Manual of Style's blog, CMOS Shop Talk, explains it with two terms:
- Restrictive (essential) clause: a relative clause is restrictive if it is essential to understanding the intended meaning of the sentence. Use "that", and no commas.
- Nonrestrictive (extra) clause: a relative clause is nonrestrictive if the information it provides is parenthetical. Use "which", set off by commas.
Chicago says "that" never introduces a nonrestrictive clause, and that in American English "which" is normally reserved for nonrestrictive clauses, which are always set off by commas. It cites sections 5.233 and 6.29 of the Manual.
The two-second test
Delete the clause and reread the sentence.
| Sentence | Delete the clause | Verdict |
|---|---|---|
| The chapter that covers pricing needs work. | "The chapter needs work." Which chapter? Unclear. | Essential: that, no commas |
| Chapter 4, which covers pricing, needs work. | "Chapter 4 needs work." Still clear. | Extra: which, with commas |
| The draft that I sent on Monday is the latest. | "The draft is the latest." Which draft? | Essential: that |
| My first draft, which I wrote in a month, needed a lot of revision. | "My first draft needed a lot of revision." Still clear. | Extra: which, with commas |
Examples written by Scribe Media.
When it changes the meaning
The commas are not decoration. Compare:
- The stories that I cut were the weakest. Only some stories were cut: the weak ones.
- The stories, which I cut, were the weakest. All the stories were cut, and by the way, they were weak.
- Clients who pay late lose their discount. Only late payers lose it.
- Clients, who pay late, lose their discount. Every client pays late, and every client loses it.
When a sentence means two different things depending on a pair of commas, that is the moment to get it right.
British usage, and what your editor will do
Chicago notes that many writers, notably in British English, use "which" in restrictive clauses too: "The chapter which covers pricing needs work." That is not wrong in British style. In American book publishing, though, a copy editor following Chicago will usually suggest "that" for essential clauses.
Chicago's advice to copy editors is that it is usually safest to get the author's or publisher's approval before changing every restrictive "which" to "that." So expect your editor to ask, then apply your answer consistently. Add your choice to your style sheet; see what a copy editor does.
For another style choice editors will ask about, see the Oxford comma.
Start here: search your manuscript for "which" without a comma before it. For each one, delete the clause and reread. If the meaning breaks, change "which" to "that."
Common questions
When do you use that vs which?
In American English, use "that" when the clause is essential to identify what you mean, with no commas: "The chapter that covers pricing needs work." Use "which" when the clause only adds extra information, set off by commas: "Chapter 4, which covers pricing, needs work."
Is it wrong to use which without a comma?
Not always. The Chicago Manual of Style notes that many writers, especially in British English, use "which" in essential clauses. In American book editing, though, "which" is normally kept for nonessential clauses with commas, and a copy editor may suggest "that" instead.
What is a quick test for that vs which?
Try deleting the clause. If the sentence still says what you mean, the clause is extra: use "which" with commas. If deleting it leaves the reader unsure which thing you mean, the clause is essential: use "that" and no commas.
Can that refer to people?
Yes, it is common: "the editor that I hired." Many writers and editors prefer "who" for people, though: "the editor who I hired" or "the editor whom I hired."
Sources: CMOS Shop Talk (The Chicago Manual of Style's blog), Commas with Restrictive and Nonrestrictive Relative Clauses (definitions; that never introduces a nonrestrictive clause; which reserved for nonrestrictive clauses in American English; British usage; get approval before changing every restrictive which; CMOS 5.233 and 6.29), read November 1, 2026. Examples and the test were written by Scribe Media. Published by Scribe Media.