That vs. Which
That and which both introduce clauses that describe a noun, but in careful American usage they mark a real distinction: that introduces a restrictive clause (one that identifies which specific thing you mean, and can't be removed without changing the sentence's meaning), while which introduces a nonrestrictive clause (extra, removable information about a thing already identified). British usage is looser about this than American style guides prescribe, which is itself a genuine, ongoing source of the confusion.
The core test: could you delete the clause?
If removing the clause changes which thing the sentence is talking about, the clause is restrictive and wants that, with no commas: The book that has a red cover is mine (implying there are multiple books, and this clause tells you which one). If the clause could be deleted without changing which thing is meant — it's just adding extra detail about something already fully identified — it's nonrestrictive and wants which, set off by commas: My favorite book, which has a red cover, is mine (I have one favorite book; the red cover is bonus detail).
Commas are not optional with which in this pattern
Because a nonrestrictive which-clause is by definition removable extra information, it should be set off by commas on both sides (or one, if it ends the sentence): The report, which took months to write, was well received. Dropping the commas around a which-clause is a frequent error that visually blurs the distinction this whole rule depends on — without the commas, a reader can't immediately tell whether the clause is meant restrictively or not, even though 'which' is technically signaling nonrestrictive.
Why American and British usage diverge here
American style guides (Chicago foremost among them) enforce the that/which split fairly strictly as a clarity convention. British usage has historically been more relaxed, using which for both restrictive and nonrestrictive clauses far more often, relying on commas alone (or context) to signal which type is intended. This divergence means a which-clause without commas isn't necessarily 'wrong' in a British-English context the way American style guides would treat it, which is worth knowing before assuming every restrictive 'which' is automatically an error.
Why the distinction matters for meaning, not just style
Because the clause type changes what a sentence claims, misusing that/which (or the accompanying commas) can create real ambiguity, not just a stylistic wrinkle: The employees who received bonuses were pleased (restrictive: only some employees got bonuses, and this clause specifies them) versus The employees, who received bonuses, were pleased (nonrestrictive: all the employees got bonuses, and this is just additional information about the whole group). Swapping the punctuation and pronoun choice genuinely changes what the sentence is asserting about who got bonuses.
Which after a preposition
Which regularly follows a preposition in either restrictive or nonrestrictive clauses, a construction 'that' can't replicate directly: the situation in which we find ourselves, the reason for which she left. In these prepositional constructions, which is required regardless of restrictiveness, since 'that' cannot directly follow a preposition in standard usage (you cannot say 'the situation in that'), making this one of the few places which appears in a genuinely restrictive role even in strict American style.
Testing your own sentences
A reliable practical test: read the sentence without the clause and ask whether it still identifies the same specific thing. If yes, the clause was extra information — use which with commas. If no — if removing the clause leaves the sentence vague about which thing is meant — the clause was essential to identifying the noun, and that (no commas) is correct.
Who instead of that or which for people
When describing people rather than things, 'who' generally replaces both that and which in most careful modern usage, though 'that' still appears for people in some restrictive contexts and is broadly accepted: the woman who called earlier (preferred) versus the woman that called earlier (acceptable, more casual). 'Which' is never used for people in standard usage regardless of restrictiveness — this is worth flagging since the underlying restrictive/nonrestrictive logic is identical across all three pronouns, only the pronoun choice itself changes based on whether the antecedent is a person or a thing.
How strict should you be about this?
Because British usage and a great deal of casual American writing don't rigidly separate that and which the way Chicago-style American prose does, treating a restrictive 'which' as an outright error in every context overstates how firmly the rule is actually followed across all English varieties. In formal American writing, though, following the distinction consistently is expected and does genuinely help a careful reader track which clauses are essential versus supplementary, which is reason enough to apply it deliberately in that register even if it's more relaxed elsewhere.
How this rule interacts with commas around 'that'
Because 'that' introduces restrictive clauses, it never takes surrounding commas — a comma before 'that' in this role is itself a signal something has gone wrong, since restrictive information by definition isn't meant to be set off as removable. If you find yourself wanting to add a comma before 'that,' it's worth checking whether the clause is actually nonrestrictive, in which case switching to 'which' (with commas) is the better fix rather than simply adding a comma while keeping 'that.'
Editing an ambiguous draft sentence
Consider a sentence drafted as: The photographs which showed the damage were released to the press. Without commas, this reads ambiguously — is 'which showed the damage' meant to specify particular photographs (restrictive, and should be 'that,' no commas), or is it added detail about photographs already fully identified elsewhere (nonrestrictive, and needs commas around it)? Resolving this ambiguity requires knowing the writer's intended meaning, then applying the that/which-plus-commas convention consistently to make that meaning unambiguous to the reader.
Style guide differences worth knowing
Chicago Manual of Style and most American editorial style guides enforce the that/which split described here fairly strictly as a matter of house convention, treating it as a tool for eliminating ambiguity. The AP Stylebook takes a similar position. Academic style guides in linguistics sometimes note, correctly, that the rule is a relatively modern (19th–20th century) editorial convention rather than an ancient or universal feature of English, since earlier English writers used which more loosely for both clause types much as British English still does today.
When 'that' can't be used at all
Beyond the restrictive/nonrestrictive distinction, 'that' simply cannot follow a preposition (you can't say 'the reason for that she left') and is generally not used for nonrestrictive clauses under any circumstance in careful usage — nonrestrictive clauses require which (for things) or who (for people) regardless of how the sentence is otherwise structured, which is why 'that' effectively has a narrower range of legitimate uses than either of its counterparts.
Why this distinction is worth the effort in careful writing
Beyond avoiding outright ambiguity, consistently distinguishing that and which signals to a careful reader, quickly and reliably, whether upcoming information is essential to the sentence's meaning or supplementary detail — a small but genuinely useful piece of structural signposting that faster, less careful writing often loses. This is one of the cases where a somewhat pedantic-feeling rule earns its keep by making longer, more complex sentences meaningfully easier to parse correctly on a first read.
Frequently asked questions
Is 'which' always wrong for restrictive clauses?
Not universally — British usage and much informal writing use 'which' restrictively without issue. In formal American style, though, that/no-commas is preferred for restrictive clauses, reserving which/commas for nonrestrictive ones.
Can a sentence be ambiguous if I get this wrong?
Yes, genuinely — swapping that for which, or adding or removing the surrounding commas, can change what the sentence claims about how many of something the clause applies to, not just its formality or polish.
Does this rule apply to 'who' as well?
The same restrictive/nonrestrictive logic applies to who-clauses describing people, using commas the same way, though 'who' itself (rather than that or which) is the standard choice for people in both restrictive and nonrestrictive positions.
Does this rule apply the same way to clauses describing entire preceding sentences?
A which-clause can also refer back to an entire preceding clause or idea rather than a single noun — She apologized, which surprised everyone — and this broader referential use is always nonrestrictive, always takes a comma, and has no 'that' equivalent, since 'that' can't take an entire clause as its antecedent this way.
Is it ever correct to use 'which' without any commas?
In careful American usage, generally no, if the clause is genuinely restrictive — that scenario calls for 'that' instead. The main exception is after a preposition (the tool with which she worked), where which is required and simply isn't set off by commas because it's functioning differently, as part of a fixed prepositional phrase rather than a separable descriptive clause. Outside that narrow prepositional case, a comma-free 'which' clause in formal American prose is usually a sign the sentence needs either a comma added or 'that' substituted in its place.