That or which?

Carrying on the theme of pairs of words, this week we are looking at that and which. These two regularly get mixed up. And then there's that bit about whether you should use a comma. It all gets a bit fraught.

You know that the aim of this blog is to make writing as simple as possible. So, let's see if we can make this easy.

Using that

Sometimes in a sentence you want to define which thing you are talking about. If you were writing about one of these two kittens you might say:

The kitten that is black and grey is my favourite.

Tough for the little ginger kitten, but you've left your readers in no doubt as to which kitten you prefer. And see how that that fits so snugly into the sentence. So, we use that for a defining clause - a clause that tells us 'which one'.

Using which

What if you were telling your friends about your new kitten and puppy? You want to tell them all about your new pets - and you already have a favourite!

The kitten, which is black and grey, is my favourite.

Now they know a little more about your favourite pet (nothing about the puppy, though). The pair of commas (after 'kitten' and 'grey') contain the extra information in a non-defining clause. You could take that clause out of the sentence and it would still make sense, we just wouldn't know as much about the kitten.

Summing it all up

Use that when you are defining which thing you are talking about. It doesn't need a comma.

Use which when you are giving extra information. Use a pair of commas to corral that extra information. If you take out the words between the commas the sentence will still make sense.

Next time someone says to you "should it be that or which?" you can casually say to them that it depends if they are using it in a defining or non-defining clause. They will be impressed.