That and which (and when to use commas)

Posted on 27 May 2015 by Lucy Gregory

 
That and which are used in many different contexts. Most of the time, there’s no confusion about which one to use. For example, in the previous sentence you’d never write ‘there’s no confusion about that one to use’.

However, both that and which can be used to introduce a description.

In the two examples below, do you know whether to use that or which and whether you need commas?

Companies ______ are incorporated in England and Wales must keep annual accounts.

Market research ______ is a long and tedious process does not guarantee success.

 

When to use ‘that’

Use that when the description afterwards is essential to the meaning of the sentence. This is the case in the first example above:

Companies that are incorporated in England and Wales must keep annual accounts.

Without the words in bold, the core meaning of the sentence changes: we go from talking about companies incorporated in England and Wales to companies generally.
 
 
Here’s another example:

The department that’s in charge of recruitment is HR.

If you removed the words in bold, the sentence wouldn’t make sense at all.

For those of you who are keen on the techy side of grammar, the part of the sentence in bold is known as a ‘defining clause’.1 This is because it defines the bit of the sentence it’s describing.

 

When to use ‘which’

You can use which in two different ways.
 

1) Use ‘which’ the same way as ‘that’ for essential information

Yup, that’s right. All the stuff I said above about that also applies to which:

Companies which are incorporated in England and Wales must keep annual accounts.

The department which is in charge of recruitment is HR.

 

2) Use ‘which’ to add non-essential information

Sometimes the information you add is not essential to the meaning of your sentence; it’s there to provide detail or comment or colour. This is known as a ‘commenting clause’:2

Market research, which is a long and tedious process, does not guarantee success.

If you remove a commenting clause from your sentence, the sentence should still make sense and its core meaning should remain the same. In the example above, the words in bold can be lifted out and the sentence reads properly: ‘Market research does not guarantee success.’

You’ll notice the words in bold sit inside a pair of commas. This is always the case. When your commenting clause comes at the end of a sentence, a full stop takes the place of the second comma:

Candidates are being interviewed by HR, which is the department in charge of recruitment.

 

Why the commas matter

Lots of people use which for both defining and commenting clauses. That’s absolutely fine because the commas make the difference.

Have a look at these two sentences:

The news was well received by the markets which had risen ahead of the election.

The news was well received by the markets, which had risen ahead of the election.

The first sentence doesn’t have a comma before which, so the information in bold is defining; it’s essential to the sentence’s meaning. This sentence tells us that some markets had risen ahead of the election but others had not. Only the markets that had risen were pleased to hear the news.

The second sentence does have a comma before which, so the information in bold is commenting; it’s not essential. This sentence says all the markets were pleased to hear the news and, by the way, all the markets had risen ahead of the election.

That single comma completely changes the meaning of the sentence.

 

Quick reminder

That or which without commas: the information that follows is essential to your sentence

Which with commas: the information that follows is not essential to your sentence
 
 
1 Defining clauses are also known as restrictive clauses.
2 Commenting clauses are also known as non-restrictive clauses.