Writing tips: That or which?

Do you sometimes wonder whether to use ‘that’ or which’ to introduce a bit of information in a sentence? If you do, you’re not alone. I hope this will help.

We have one toilet which is out of action.
 
So said the announcer on a train I’d just boarded recently. What did he mean? Was there enough of a pause after ‘toilet’ to suggest a comma? Like this:
 
We have one toilet, which is out of action.
 
– meaning there was only one toilet, and it was out of action.
 
Or, less worryingly with a longish journey ahead, was there no discernible break in his speech? If there wasn’t, his announcement could have been interpreted as:
 
We have one toilet that is out of action.
 
– in other words, one of their toilets was out of action.
 
It soon became clear that he meant the latter. But why make us fret, even for 30 seconds?
 
Now, I’m not in the business of copy-editing the spoken words of railway staff – or anyone else – but this is a fine example of the difference between those two commonly confused words: ‘that’ and ‘which’. Some people think they’re interchangeable in this context, and there is some truth in that – but you should take care when using them, and, as we’ll soon see, when deciding whether to use a comma before your chosen word.
 
Grammatical lingo alert
 
When ‘that’ or ‘which’ is used in this way, it’s introducing either a restrictive (or defining) or a non-restrictive (or non-defining) clause. Understanding the difference between these is the key to getting this right.
 
A restrictive clause restricts (or defines) the scope of what we’re writing about. A non-restrictive clause, on the other hand, doesn’t do this – it just adds some information.
 
Let’s look at another example:
 
(a) They painted the doors that had just been fitted.
(b) They painted the doors, which had just been fitted.
 
Which doors did they paint? In (a), the statement applies to only some of the doors: the ones that had just been fitted. There might be other doors as well, but if so, the ‘that ...’ clause is excluding them from the statement – that’s why we call it a ‘restrictive’ clause.
 
The statement in (b), meanwhile, applies to all of the doors. The clause beginning with ‘which’ is just telling us something extra: ‘also, the doors had just been fitted’. It doesn’t restrict the scope of what we’re referring to, so it’s a non-restrictive clause. The first four words would be meaningful and correct by themselves, though less informative.
 
Careful with commas
 
The other key difference between (a) and (b) above is the use or non-use of a comma after ‘doors’. A comma – or a pause in speech – tells us that the clause is non-restrictive.
 
Similarly, if the clause is in the middle of a sentence, it should also be followed by a comma only if the clause is non-restrictive:
 
(a) The doors that had just been fitted were going to be painted.
(b) The doors, which had just been fitted, were going to be painted.
 
In (a), the clause beginning with ‘that’ is integral to the statement, so it shouldn’t be cordoned off with punctuation.
 
In contrast, the commas in (b) are used to enclose supplementary information; these are known as parenthetical commas. Parentheses (round brackets) or dashes could be used instead, though each of these three choices has a different effect in terms of emphasis.
 
Decision time
 
Now we come to the crunch. With a non-restrictive clause, ‘which’ should always be used, preceded by a comma. So we can’t have this, which would probably baffle the poor reader:
 
They painted the doors, that had just been fitted.
 
Instead, we need this, as we saw earlier:

They painted the doors, which had just been fitted.

With a restrictive clause, though, either ‘that’ or ‘which’ is considered acceptable, and it all hinges (following the door theme) on the presence or absence of a comma. These two sentences mean the same thing:
 
(a) They painted the doors that had just been fitted.
(b) They painted the doors which had just been fitted.
 
A verdict
 
So, does it matter whether we use ‘that’ or ‘which’ with a restrictive clause?
 
Some argue that it doesn’t, but I’m firmly in the ‘that’ camp. It’s common sense, not dogma: you can’t go wrong with ‘that’, but you can create at least a flicker of confusion by using ‘which’ – so why use it? Although the lack of a comma technically shows that the clause is restrictive, some readers might not be fully aware of this rule, or might not notice whether there’s a comma or not.
 
I would only make an exception where another ‘that’ is lurking nearby, just to reduce repetition, as in this:
 
They used red paint for all of the doors which weren’t already that colour.
 
At the risk of riling purists, though, I’d like to argue that it sometimes doesn’t particularly matter whether the clause is restrictive or not. So we could use any of the three forms, like here:
 
(a) They moved into a house that had just been renovated.
(b) They moved into a house which had just been renovated.
(c) They moved into a house, which had just been renovated.
 
Whichever version we use, there’s clearly only one house involved, and it’s clearly just been renovated – no ambiguity, and no real need to ponder over which version is best.
 
Not only but also …
 
The same issues arise with some other words and expressions, such as ‘who’, ‘where’ and ‘in which’. For example:
 
(a) She praised the staff who had worked hard.
(b) She praised the staff, who had worked hard.
 
In these cases, we don’t have a choice of wording, and the humble comma can make a big difference, as it does above. We can only hope that the reader will understand the difference it makes.
 
Summing up
 
As usual in writing and editing, we should be helping the reader by making things as clear and unambiguous as we can.
 
Just because a word is ‘acceptable’ in the context, that doesn’t make it a good choice. This applies to the ‘that’ vs ‘which’ debate as much as to anything else.