A simple post-it note trick to use ‘that’ and ‘which’ correctly

A PhD student recently asked me how to use ‘that’ and ‘which’ correctly. I have to think really carefully when I explain the correct use of these words.

At school in the UK, I was taught the British English style for using ‘that’ and ‘which’.

However, most scientific journals/style guides use the American style—which is the exact opposite of the British style! As if English wasn’t difficult enough…

Image from memegenerator.net

I thought this would be a good time for some revision! So, I made another cup of coffee, dusted off my AMA Manual of Style and browsed the internet.

Here’s what I learned (again):

‘That’ is used for clauses that are restrictive.
‘Which’ is used for non-restrictive clauses, which should be separated using commas.

These definitions turned out to be “happy” accidents, because each definition actually uses the word it defines correctly. Let’s look at these definitions in more detail.

1. ‘That’ is used for clauses that are restrictive.

To use grammar jargon, we use ‘that’ for restrictive clauses. A restrictive clause is a part of a sentence that provides essential information needed to identify or specify the noun that comes before ‘that’.

For example: I bought the cake that I like best. There might have been lots of cakes in the shop, but I only bought my favourite cake (it’s raspberry and vanilla, if you’re wondering!).

2. ‘Which’ is used for non-restrictive clauses, which should be separated using commas.

In simple terms, a non-restrictive clause is some information that is not essential to identify or specify the noun before ‘which’. The underlined which provides extra, non-essential information and is separated by commas.

For example: I ate the cake, which was delicious. The important news is that I ate the cake. Sorry, there’s none left!

In this scenario, there was probably only one cake in the house, so we don’t need to use a restrictive clause to identify or specify the cake I ate.

The fact I found the cake delicious is extra detail that isn’t needed to understand that the cake is gone or to know which cake I ate.

Saying I ate the cake that was delicious would incorrectly imply there was more than one cake, I tasted all of the cakes (naughty!), but only one cake was delicious and I ate it.

These phrases make it much easier to explain the correct use of these words! So, I’ve written my definitions on a post-it note and stuck it on my pencil tin.

Definitions of That and Which on post-it note

So, we could probably use both ‘that’ and ‘which’ correctly by remembering these definitions!

An example of ‘that’/’which’ at work in scientific writing

Everything is a little more complicated in scientific writing, so let’s look at one example to see ‘that’ and ‘which’ in practice.

Which of these sentences do you think is correct?

(i) Osteoclasts are important for bone resorption as they secrete protons that decrease the pH and thus solubilize the mineral phase.

(ii) Osteoclasts are important for bone resorption as they secrete protons, which decrease the pH and thus solubilize the mineral phase.

This is a really tricky question. I had to ask myself - is ‘decrease the pH and thus solubilize the mineral phase’ essential to understand or specify protons (the noun)?

I think ‘decrease the pH and thus solubilize the mineral phase’ isn’t essential, but it does provide some nice additional detail. If we remove this clause, then ‘Osteoclasts are important for bone resorption as they secrete protons’ still makes sense on its own.

We’ve decided the clause is not essential/non-restrictive, so we should use ‘which’ and a comma. Therefore, option (ii) is correct.

Taking this from another perspective, using ‘that’ / option (i) actually suggests there are different types of protons: some protons that decrease the pH, and some protons that don’t affect the pH. Remember ‘that’ makes the following clause seem like a definition of/for the preceding noun (in this case, protons).

These phrases make it much easier to explain the correct use of these words!

As a final point, sometimes you can encounter a sentence where it is difficult to decide if ‘that’ or ‘which’ is appropriate. In these cases, the best option is to split the sentence into two or rearrange the words. For our example, we could write something like either of these options:

Osteoclasts are important for bone resorption. The protons secreted by osteoclasts decrease the pH and thus solubilize the mineral phase.

Osteoclasts are important for bone resorption as the protons secreted by osteoclasts decrease the pH and thus solubilize the mineral phase.

Have you experienced similar confusion with ‘that’/’which or another set of words? Or do you know a better way to remember how to use these words correctly?

Email andrea@scienceeditingexperts.com and tell me; I’d love to know!

To your success,

Andrea

Welcome!

At Science Editing Experts, we help scientists like you to submit well-written journal papers with confidence and complete your thesis without headaches, so you can focus on your research and career.

Andrea Devlin PhD

Chief editor and owner of Science Editing Experts

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