That which confuses

There's a common editorial house style rule, often irritating to writers, that the uses of the relative pronouns 'that' and 'which' are distinct. Here are examples of those uses.

• We walked down the road that was covered with Tarmac and turned right
• We walked down the road, which was covered with Tarmac, and turned right.

So the main difference is that 'which' takes commas and 'that' doesn't, right?

Not quite. The main difference is that example A includes a defining clause. The object of the sentence, the road, is distinguished by having Tarmac. To put it another way, while all the other roads at the crossroads didn't have Tarmac, the one we are interested in, the one we are walking on, definitely does.

By contrast, sentence B includes a non-defining clause. In this sentence there was no imaginary crossroads, so there's need to define which path we took; we're just adding the interesting fact that our road happens to have Tarmac slapped all over it.

Some editors who don't mind occasionally irritating writers will insist on hunting out all 'which-es' in defining clauses. That's a bit overzealous in my opinion. In many instances you can use either 'that' or 'which'. To modify example A:

• We walked down the road which was covered in Tarmac and turned right.

'Which' can introduce ambiguity, as it does here (was it the only road or the only road that was covered in Tarmac?). But not always and, even if it does, a slight sense of uncertainty might be intentional. It's the editor's job to work out whether replacing the 'which' with 'that' really is justified or not, not to slap down a blanket ban on the use of 'which' in defining clauses.

So what about non-defining clauses? These clauses are descriptive agents, not necessary ones. In this situation we often make sure that there is a comma, because the comma does a lot of grammatical grunt work in making it clear to the reader that what's coming next is a nice add-on and not integral to the construction of the sentence.

The relative pronoun 'that' does not work with descriptive clauses; only with defining ones. Which is why it never looks right after a comma. To modify example B.

• We walked down the road, that was covered in Tarmac, and ...

So a modified house style rule would be: "always use 'which' for non-defining clauses, and consider the use of commas, and use either 'that' or 'which' with defining clauses.

In American English, the rule is slightly different. The American way is to use 'which' only for non-defining clauses. But the Americans do things differently with commas too. More on that another time.