By Mary Morel
With which and that used to introduce clauses, we’re talking about restrictive and non-restrictive clauses, sometimes called defining and non-defining clauses.
Some people think of which as describing and that as defining.
An example will make this clearer.
The bike that is in the shed needs fixing. (defines) There may be another bike somewhere else, but it is the one in the shed that needs fixing.
The bike, which is in the shed, needs fixing. (describes)
You can delete the information in the brackets and the sentence still makes sense.
The bike needs fixing.
These days, grammar experts think you can use which or that interchangeably to define.
The bike which is in the shed needs fixing.
The bike that is in the shed needs fixing. (my preference)
When you are using which to describe, you need commas around the whole clause. We have no commas with which or that when they are defining.
This is the house which/that Jack built.
If you follow this interpretation, your choice of whether to use which or that will often depend on the formality of your writing — which sounds more formal than that.
Occasionally, using the traditional rules adds 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 goes on to say: ‘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?’
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