He said that that 'that' that that man used was wrong.

If you understand this sentence, you probably don't need to read on. If you don't, you could be one of the many who are a bit hazy on how and when to use the word 'that'. It's a versatile little beast, with many uses. The one that people most often get wrong is the use of 'that' as opposed to 'which'.

Here at Right Words, we engage in 'which' hunts, because what usually happens is that people use 'which' when they should use 'that'.

The difference is this. 'Which' introduces relative clauses - ones that give additional information about something in the sentence, and that can be added or removed without affecting the statement made in the main clause.

The exhibition, which was held in the market reserve, contained some interesting examples of contemporary sculpture.

In this example, the writer assumes that the reader knows which exhibition is being talked about, and the statement of the venue is just an aside.

'That', on the other hand, introduces clauses that identify the thing they're attached to.

The exhibition that was held in the market reserve contained some interesting examples of contemporary sculpture.

Here, there is no assumption that the reader knows which exhibition is being talked about. The writer is telling the reader which one it is: the one in the market reserve (and not the one in the Botanic Gardens or the one in the Town Hall).

Because 'that' clauses have this identifying function, they can't be taken away without affecting the statement made in the main clause. They form a sense unit with the word they're attached to, and that's why they aren't preceded by a comma.

So, for our opening sentence, a lot of people would say: He said that that 'that' which that man used was wrong ... but they'd be wrong - and the man was probably right!