That vs Which: The Rule, With Examples
If the clause is needed to identify the thing, use that and no commas. If it only adds extra information, use which between commas.
The report that I sent is late. The report, which I sent on Monday, is late.
The test that settles it
Take the clause out. If the reader can still tell which thing you mean, the clause was an aside — use which and comma it off. If removing it leaves the sentence vague, the clause is doing identifying work, and that is the safer choice.
“the house ___ burned down” → without it you do not know which house, so that.
“our office, ___ opened in May, is closed today” → you already know the office, so which.
Commas and which travel together. If you would not pause when reading it aloud, you probably do not want which.
That: the clause that identifies
It introduces a defining clause — one the sentence needs in order to say which thing it is talking about. No commas, ever, and the clause cannot be lifted out without loss.
The train that leaves at six is fastest.
Anything that can go wrong will.
the only book that mattered
Would removing the clause leave the reader asking “which one?” Then that.
Which: the clause that comments
It introduces a non-defining clause — an aside about something already identified. It is fenced off by commas, and the sentence survives without it.
The train, which was packed, left late.
Her flat, which she rents, is tiny.
He resigned, which surprised nobody.
Could the clause sit in brackets instead? Then which, with commas.
Side by side
| That | Which |
|---|---|
| Identifies which one | Adds an aside |
| No commas | Commas on both sides |
| Cannot be removed | Can be removed |
| the cake that she made | the cake, which she made, |
The exceptions
Reserving that for every defining clause is a convention promoted by American style guides. British writers have used defining which for centuries — the report which I sent is late is not an error, and editors outside the United States will not mark it as one.
What is a real rule, and universal: a clause set off by commas never takes that, and a defining clause never takes commas. The punctuation carries the meaning; the word choice is largely style.
You can write the house in which he lived but never the house in that he lived. If a preposition comes first, only which will do — or move the preposition to the end and use that: the house that he lived in.
For people, prefer who. That is acceptable for people and always has been, but who reads better.
Where people get it wrong
| Incorrect | Correct | Why |
|---|---|---|
| The car, that I bought last year, has broken down. | The car, which I bought last year, has broken down. | Commas force which. |
| This is the report which you asked for, not that one. | This is the report that you asked for, not that one. | It identifies which report. |
| the meeting in that we agreed the budget | the meeting in which we agreed the budget | After a preposition, only which. |
| He missed the deadline which annoyed everyone. | He missed the deadline, which annoyed everyone. | The aside needs its comma. |
Practice — choose the right word, then check yourself.
- The email ___ you forwarded never arrived.
- Our head office, ___ moved last year, is in Leeds.
- the desk on ___ the vase stood
- She took the job ___ paid least.
- They cancelled the show, ___ upset the cast.
Show the answers
1. that 2. which 3. which 4. that 5. which
Frequently asked
Is defining which really acceptable?
In British English, yes, and it has a long pedigree. In American editing it will usually be changed to that.
What matters most?
The commas. Getting those right conveys your meaning even if the word choice follows a different house style.
More words people confuse
