This is one that confuses a lot of people. For many, ‘that’ and ‘which’ seem completely interchangeable, and it can be confusing to find that in some cases they are not. Not to worry, I am here to help! Once you understand the reasons behind the differing uses, it’s actually pretty easy to work out what you need to do.
It all comes down to restrictive and non-restrictive clauses. ‘That’ is always restrictive, but ‘which’ is non-restrictive (usually).
Restrictive and non-restrictive clauses
‘But what the hell is a restrictive clause?’ you may be thinking. Restrictive clauses restrict the type of thing (noun, or class of nouns) being talked about. By contrast, non-restrictive clauses simply provide some additional information.
Take the following examples:
Moon rocks that are really made of cheese are yellow in colour.
Moon rocks, which are really made of cheese, are yellow in colour.
The first sentence contains a restrictive clause, ‘that are really made of cheese’, telling us that the ‘moon rocks’ being referred to are only those moon rocks that are made of cheese. Other moon rocks may or may not be yellow, but we’re not talking about them. We have restricted the scope of the term ‘moon rock’ to refer to only cheese-based moon rocks.
By contrast, the second sentence is saying something much larger in scope. Because ‘which are really made of cheese’ is non-restrictive, ‘moon rocks’ is unlimited in scope. We are talking about all moon rocks and asserting that they are all yellow, and merely adding the additional information that moon rocks are made of cheese.
Punctuation helps us out. Non-restrictive clauses function as ‘asides’ or parenthetical clauses – you could remove them without changing the meaning of the main (independent) clause – and this is indicated by the use of parenthetical commas: the one before ‘which’ and the one after ‘cheese’. We don’t use these with the restrictive clause because it would change the meaning of the sentence if you were to remove ‘that are really made of cheese’ – the scope of ‘moon rocks’ would no longer be limited to cheese-based moon rocks. Far more moon rocks would need to be yellow in order for the sentence to be true.
So, it’s actually a lot easier than it sounds. If you’re just adding some additional information about the thing (noun or class of nouns) you’re talking about, use ‘which’. If the extra information you’re adding restricts the meaning of the thing (noun or type of nouns) you are talking about, use ‘that’.
Using ‘which’ in a restrictive clause
Where it gets confusing is that some style guides allow for the use of ‘which’ in a restrictive clause. Oxford Dictionaries, for instance, says that both are correct. Fowler’s Modern English Usage bemoans that the distinction has not been neatly preserved, but concedes that both ‘which’ and ‘that’ are used for restrictive clauses. By contrast, Grammar Girl is very clear that ‘which’ should only be used for non-restrictive clauses.
I’m amused to have seen both US and UK grammar guides accuse the other of being more eager to exclude ‘which’ from restrictive clauses – beware of anyone who says of a grammar rule ‘Oh, it’s an Americanism’ or ‘Oh, the British do that, but we don’t’ – they’re often using the specter of trans-Atlantic conflict to support a personal prejudice. At the end of the day, there’s nothing strictly wrong with using ‘which’ in a restrictive clause, and it’s not a new, strictly British, or strictly American, habit. Fowler’s is nice for this, because it goes into a bit of history of usage, rather than simply asserting a rule, and we have this nice quote from the 1926 edition:
The relations between that, who, and which, have come to us from our forefathers as an odd jumble, and plainly show that language has not been constructed by a master-builder
– quoted in R W Birchfield (1996), Fowler’s Modern English Usage, Third Edition, p. 774
Here’s what I think about it:
As a writer, one’s duty is to convey one’s ideas to the reader in the most effective way possible, which usually means as clearly as possible. Why not stick to ‘that’ for restrictive clauses and ‘which’ for non-restrictive clauses? It saves your reader the trouble of wondering if you maybe made a mistake and meant the one thing rather than the other.
As a proofreader/copy editor, my duty is to the publication. I’ll follow whatever rule they prefer. And it might surprise you. Academic publications often prefer to respect authorial choice on the basis that the writer is themselves treated as an authority. By contrast, fiction publishers often don’t say anything on the matter, or prefer whatever makes the reading experience smoother, so sometimes prefer to keep only to ‘that’ for restrictive clauses. But if I haven’t been told otherwise, I just leave it be if it’s permissible.
One thing to be clear on, though: one should never use ‘that’ in a non-restrictive clause. In the simplest possible terms: if there needs to be a comma before it, you should only ever use ‘which’. Or: if it’s an aside, not integral to the sentence, always use ‘which’ and always put a comma before it.
I hope this helps to clarify things. Once you get the hang of it, it’s dead easy, I promise. In the meantime, you can refer back to this guide when in doubt 🙂 .
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