Is There a Difference Between ‘That’ and ‘Which’?

The short answer is yes, there is a difference.

Ready for the long answer now?

Good. They're actually a pair of words that are often mixed up and increasingly, they're being used interchangeably. But there is a difference, and I think it does matter. It can change the whole meaning of the sentence. But how, I hear you cry. Actually, it's not that complicated. It may sound it at first, but bear with me...

Restrictive and non-restrictive clauses

You look in any style guide or grammar book, and you may find yourself bamboozled by technical terms. Like this:

That goes with restrictive relative clauses, and which goes with non-restrictive relative clauses.

Huh?

The Chicago Manual of Style says 'that' is used to "narrow a category or identify a particular item being talked about," whereas 'which' should only be used when "preceded by a preposition." (see 5.250)

Double huh?

Let's break it down.

Restrictive clauses

A restrictive clause is necessary for the sentence to make sense, and it provides key information that is essential.

The woman who lives next door has just had a baby.

In the above sentence, 'who lives next door' is restrictive because, without it, the sentence would become, the woman has just had a baby.

The type of food that my father cooks makes me feel sick.

'That my father cooks,' in this sentence, is imperative for its meaning, making it a restrictive clause. The type of food makes me feel sick doesn't work as a sentence (although, admittedly, the whole sentence could have been written better in the first place... my father's cooking makes me feel sick.)

Note that, in these types of sentences, the word 'that' can sometimes (but not always) be removed completely - and keeping an eye out for those instances will help improve your writing!

Non-restrictive clauses

A non-restrictive clause is an addition to the sentence, some extra information.

Betty, who lives next door, has just had a baby.

The non-restrictive clause, 'who lives next door,' is not required to make the sentence make sense. Here's another:

The book, which is my favourite, has a blue cover.

Non-restrictive clauses (and therefore, sentences with 'which' in them) tend to be bracketed with commas, making them much easier to spot.

When it makes a difference

It still might seem like it makes no difference, but there are certain times when it changes the meaning. Take these two sentences as an example:

Our car that has two doors is red.
Our car, which has two doors, is red.

The first sentence means that we have more than one car, and that the one with two doors is red. The fact that we're talking specifically about the two-door car is imperative to the meaning of the sentence.

The second sentence means there is one car that just happens to have two doors. The fact that it is a two-door car is additional information that is not required.

A difference in nations

Surprisingly, while we generally think of British English as being more pernickety, this difference is much more pronounced in American English. All the major American style guides advise on using the correct term. In British English, it's a little more lax. The New Oxford Style Manual, for example, says "note that in restrictive relative clauses either which or that may be used in British English, but in non-restrictive relative clauses only which may be used." (see 4.3.1)

For me, I think that confuses the issue unnecessarily, and it's probably this that has contributed to the words being used interchangeably. It's not often I say this, but I think I'll stick to the American way!

An easy way to remember

Ask yourself the question: does eliminating the clause change the meaning of the sentence, or does it only add extra information? If it's the former, use that. If it's the latter, use which. If you're still not sure, try re-writing the sentence completely and see what you come up with.

Believe me, after a couple of times, it'll start coming completely naturally to you!