Use that to introduce a restrictive clause — the information a sentence needs to pin down what you mean — and add no commas. Use which to introduce a non-restrictive clause — extra detail you could drop — and wrap it in commas. The one rule nobody breaks: a clause set off by commas always takes which, never that.
The quick rule
- Restrictive (essential): use that, no commas. The keys that open the front door are missing.
- Non-restrictive (extra): use which, with commas. My house keys, which cost £40 to cut, are missing.
If you can remove the clause and the sentence still points to the same thing, it is non-restrictive: use which and commas. If removing it leaves you asking "which one?", it is restrictive: use that and no commas.
What "restrictive" and "non-restrictive" actually mean
A restrictive clause (also called a defining clause) narrows down the noun. It tells you which person or thing you are talking about, so you cannot delete it without changing the meaning:
- Books that are printed on acidic paper eventually yellow. — only acid-paper books, not all books.
A non-restrictive clause (a non-defining clause) adds a side note about something already identified. Lift it out and the core sentence survives intact:
- This book, which I bought in Rome, is printed on acidic paper. — you already know which book; its origin is a bonus fact.
Here is the part most guides bury: the commas are the real grammar, not the word. In speech you can hear it — a non-restrictive clause comes with a small pause on each side and a dip in the voice; a restrictive clause is spoken without a break. The commas simply write that pause down.
See how the commas change the meaning
Move the commas and you change the fact being stated. All of these are grammatical, but they do not mean the same thing:
- The paintings that were fake were removed. — Some paintings were fake; only those went. (restrictive)
- The paintings, which were fake, were removed. — Every painting was fake, and all of them went. (non-restrictive)
Another pair:
- My sister who lives in Leeds is a nurse. — I have more than one sister; I am identifying the Leeds one. (no commas)
- My sister, who lives in Leeds, is a nurse. — I have one sister; where she lives is just extra colour. (commas)
Those last two use who because the subject is a person, but the comma logic is identical.
The mistakes that are genuinely wrong
Two errors count as errors on both sides of the Atlantic:
- Using that after a comma. ✗ The Shard, that opened in 2012, dominates the skyline. → ✓ The Shard, which opened in 2012, dominates the skyline. A non-restrictive clause can never begin with that.
- Putting commas around a restrictive clause. ✗ People, who live in glass houses, shouldn't throw stones. → ✓ People who live in glass houses shouldn't throw stones. The commas here falsely claim the saying is about all people.
The "rule" that is really just a style preference
You may have been taught that restrictive clauses must take that and that a restrictive which is an error. That is not a law of English grammar — it is a house style, mostly American, and a fairly recent one. The house which Jack built has been standard English for centuries and is completely correct, especially in Britain.
The convention was proposed by H. W. Fowler in 1926, who tidily suggested reserving that for defining clauses and which for non-defining ones — then honestly admitted that "it would be idle to pretend that it is the practice either of most or of the best writers." In other words, even the man who popularised the rule knew good writers ignored it.
So keep two things apart:
- Non-negotiable grammar: non-restrictive clauses take commas and which (or who), never that; restrictive clauses take no commas.
- Optional style: whether a restrictive clause uses that or which. Both are correct; American editors prefer that.
US vs UK: where it actually matters
- American English enforces the split. The Chicago Manual of Style, the AP Stylebook, and usage authority Bryan Garner all flag a restrictive which and change it to that. If you write for a US publication, use that for restrictive clauses.
- British English is relaxed about it. The report which was late is unremarkable in the UK, and British style guides do not insist on the swap.
What both varieties agree on is the punctuation. Get the commas right and no reader on either continent will fault you.
Tricky cases and exceptions
- After a preposition, use which. You cannot say the drawer in that I keep receipts. Write the drawer in which I keep receipts, or move the preposition to the end: the drawer that I keep receipts in. (Ending a sentence with a preposition is fine — that is another myth.)
- Which can stand for a whole clause. The 8:15 was cancelled, which ruined my morning. Here which refers to the entire event; that cannot do this.
- You can drop the pronoun entirely. When the relative word is the object of a restrictive clause, cut it: the film I watched works as well as the film that I watched. Keep it when it is the subject: the film that won the award.
- Prefer that after superlatives and words like all, only, every, any, nothing. The best advice that I ever got; everything that could go wrong did. Which sounds stilted here.
- For people, use who. The woman who phoned reads better than the woman that phoned, although that for people is not strictly wrong. Never use which for a person.
A memory trick
Use the bracket test. If you could drop the clause into parentheses — or slip "by the way" in front of it — without breaking the sentence, it is non-restrictive: reach for commas and which. If cutting it leaves the reader asking "which one?", it is restrictive: no commas and that. Or just remember the pairing: commas and which travel together; that works alone.
