By Mary Morel
When which and that are used to introduce clauses, we’re talking about restrictive and non-restrictive clauses, sometimes called defining and non-defining clauses.
Take a look at these two sentences.
The bike, which is in the shed, needs fixing. (non-restrictive or non-defining)
The bike that is in the shed needs fixing. (restrictive or defining)
First sentence (which)
In the first sentence, the bike needs fixing, and, by the way, it’s in the shed (‘which is in the shed’ can be deleted and the sentence still makes sense).
The bike needs fixing.
When you are using which to add extra information, you need a comma at both ends of the clause.
The hat, which I wore to the wedding, was new.
Second sentence (that)
In the second sentence, there may be other bikes on the property, but it is the bike in the shed that needs fixing (‘that is in the shed’ is essential for the meaning of the sentence).
Can you ignore the traditional rules about which and that?
In practice, many of us use which and that interchangeably with restrictive clauses.
This is the house that Jack built.
This is the house which Jack built.
The bike that is in the shed needs fixing.
The bike which is in the shed needs fixing.
Your choice of which or that will often depend on the formality of your writing — which sounds more formal than that.
Occasionally the rules about which and that matter
The traditional rules can aid clarity. The Australian Commonwealth Style Manual gives this example.
The research findings that were likely to cause embarrassment were never circulated.
The research findings which were likely to cause embarrassment were never circulated.
The research findings, which were likely to cause embarrassment, were never circulated.
It says: ‘The first example makes it clear that the research findings not circulated were the ones likely to cause embarrassment. In the third example, it is plain that none of the recommendations was circulated. The situation described in the middle example is ambiguous: were all of the findings withheld or just the embarrassing ones?’