Science, Technology and Engineering

Academic Language and Learning

'That' and 'which'

Using the relative pronouns that and which can be confusing. Some writers would argue that it doesn’t particularly matter which you use and that it is appropriate to use the pronoun that sounds best in the context. Other writers argue that it does matter because it affects meaning.

If you (or your supervisor) think that it matters, then how do you decide when to use the two pronouns?

Simple answer

• If you need a comma, use which
• If you don’t need a comma, use that

More complex answer

For example:

The epidemiological study, which involved three hundred communities, was not well received.

The point of this sentence is that the epidemiological study was not well received – that it happened to involve three hundred communities is a bit of extra information. The information contained between the commas is parenthetical – it is not essential to the main point of the sentence. Thus it is a non-restrictive clause, follows a comma and begins with which.

The following sentence has quite a different meaning.

The epidemiological study that involved three hundred communities was not well received.

Unlike the situation in the first example, here the fact that the study involved three hundred communities is central to the meaning of the sentence. The clause that involved three hundred communities tells us which epidemiological study we are talking about. Thus it is a restrictive clause, requires no commas and begins with that.