When editing almost any piece of writing, chances are that the writer will have made a few mistakes in the choice between ‘that’ and ‘which’. So often, I need to turn a ‘which’ into a ‘that’, as the difference is a tricky one to grasp. This little conundrum seems to appear again and again, so it sits on my mental list of favourite features to correct. The distinction can cause ambiguity, so it is worth getting it right. It takes just a little practice for it to become second nature.
Use ‘comma which’ to add a separate clause
We’ll start with the correct way to use ‘which’. The best use of this word is to follow a clause that could be complete in itself, but about which you want to add some extra information. You need a comma before the ‘which’ in this case. Here’s an example:
My painting was selected, which made me very happy.
The first clause is in red so you can see that it could stand alone. ‘My painting was selected’ is a sentence in its own right. We are adding the information that its selection has made me very happy. You could also say ‘My painting was selected and this made me very happy’.
Use ‘that’ to define from within a set
Now, if you want to restrict or define the painting, you would use ‘that’. You want to specify the exact painting out of a set of all possible paintings. Consider this example:
My painting that I did on the weekend was selected.
In this case, the information before the ‘that’ cannot stand alone. We cannot substitute ‘and this’, or make it into the new sentence ‘My painting’, because we have only a subject. It needs more; we are specifying which painting, and what happened with it.
What’s wrong with using ‘which’ in a defining clause? What’s the difference?
Now, you might ask, why can’t we simply use ‘which’ for this last example sentence as well. What’s the difference? Of course many people do, making the sentence ‘My painting which I did on the weekend was selected’. This is technically not incorrect. There are, however, situations where using ‘which’ instead of ‘that’ could be ambiguous. Continuing along the arty theme, look at these sentences:
- The pictures that were painted on the weekend were selected. Good!
- The pictures which were painted on the weekend were selected. OK, but not advisable.
- The pictures, which were painted on the weekend, were selected. Good!
Sentence 1: There was a set of pictures, and some of them were painted on the weekend. These ones were selected.
Sentence 2: Could be ambiguous. Either as in sentence 1, some were painted on the weekend, and these were selected, or all were painted on the weekend and all were selected.
Sentence 3: The addition of the comma clearly shows that all pictures were selected, having all been painted on the weekend.
So you see, keeping ‘which’ to add non-defining information to follow stand-alone clauses makes sense. The difference is that it reduces ambiguity. Use ‘that’ rather than ‘which’ to narrow to a particular member of a set. There’s a nice explanation in The Guardian – perhaps neater than mine, so if you’re still confused, have a look at that. Or you can contact me for help with your text!