Which vs that
The short answer
That introduces a clause that identifies which one you mean, and takes no commas: the report that we discussed. Which introduces extra information, and takes commas: the report, which ran to sixty pages. The commas carry the meaning; the pronoun follows them.
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At a glance
That or Which?
| Compare | ThatDefines which one. No commas. | WhichAdds detail. Commas. |
|---|---|---|
| Job | Says which one you mean | Adds information about a known one |
| Commas | None | Always a pair, or one at the end |
| Can it be omitted? | Yes, when it is the object | Never |
| People | that or who | who, not which |
| After a preposition | Not used: move it to the end | Yes: in which, for which |
| Removing the clause | Changes the meaning | Leaves the meaning intact |
How to choose
Ask these, in order
- 1
Does the clause tell the reader which one you mean?
That, with no commas.
The email that arrived this morning was from legal.
Identifies which email out of several.
- 2
Is the thing already identified, and you are adding a detail?
Which, inside commas.
The email, which arrived this morning, was from legal.
There is one email; the timing is extra.
- 3
Can you delete the clause without changing which thing you mean?
Which, with commas. If deleting it leaves the sentence vague, use that.
Cars that run on diesel are being phased out.
Only diesel ones, deleting the clause changes the claim entirely.
- 4
Is there a preposition in front?
Which. That cannot follow a preposition, so either use which or move the preposition to the end.
the meeting at which it was decided / the meeting that it was decided at
Formal, then ordinary.
Side by side
Each one, in full
Defining clauses: that
Defines which one. No commas.
A defining clause narrows down which thing you are talking about, so it is essential to the sentence and takes no punctuation. The book that I lent you identifies one book among several.
When the relative pronoun is the object of its clause, English lets you drop it entirely: the book I lent you. That option exists only for defining clauses, which is a useful secondary test: if you can delete the word, it was a defining clause.
The flat that we looked at last week has gone.
Which flat, the one we saw.
Everything you said was right.
The that has been dropped.
The only thing that matters is the deadline.
After only, superlatives and all, that is strongly preferred.
Non-defining clauses, which
Adds detail. Commas.
A non-defining clause adds information about something already identified. It sits inside commas and can be lifted out without damaging the sentence: my flat, which is on the third floor, is tiny.
Which can also refer back to a whole clause rather than a single noun: he arrived two hours late, which explains the mood. That construction always takes a comma and always uses which, never that.
The Mistral, which blows from the north, can last for days.
One wind, extra detail.
She missed the train, which meant a two-hour wait.
Which refers to the whole clause.
Our head office, which moved last year, is now in Lyon.
There is one head office; the move is incidental.
Mistakes to avoid
What learners actually get wrong
The report, that we discussed, was rejected.
The report that we discussed was rejected. / The report, which we discussed, was rejected.
That never takes commas. Both repairs are correct English but they say different things, one identifies which report, the other adds a detail about the only one.
My brother, that lives in Oslo, is visiting.
My brother, who lives in Oslo, is visiting.
Non-defining clauses about people take who. That is possible in defining clauses about people, the man that called, but never inside commas.
The house in that we grew up has been sold.
The house in which we grew up has been sold.
That cannot follow a preposition. Either use which, or move the preposition to the end: the house that we grew up in, which is what most speakers actually say.
Dogs, which bark at night, should be kept indoors.
Dogs that bark at night should be kept indoors.
With commas the sentence claims all dogs bark at night. The punctuation is doing the semantic work here, and getting it wrong produces a sentence that is grammatical and false.
Now use it
Relative clauses
who, which, that, whose, and when you can drop the pronoun. Reading a rule and being able to apply it are different skills, and only one of them shows up when you speak.
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Questions
Asked and answered
- Is “which” in a defining clause actually wrong?
- Not in British English, where the book which I lent you is perfectly standard. The strict that-for-defining rule is largely an American editorial convention. What is not optional in either variety is the comma rule, commas always mean non-defining.
- When can I drop the relative pronoun?
- Only in a defining clause, and only when the pronoun is the object: the film we saw. If it is the subject you must keep it, the film that won cannot become the film won. Many other languages never allow the omission at all.
- What about who and whom here?
- They follow the same defining and non-defining split, with who for the subject and whom for the object. The comma rule is identical, so if you have this pair, who and whom only adds the subject-object choice on top.