Which and that

The bike, which is on the footpath, needs fixing.
The bike that is on the footpath needs fixing.

The difference between which and that

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 on the footpath, needs fixing. (non-restrictive or non-defining)

The bike that is on the footpath needs fixing. (restrictive or defining)

Which adds extra information

In the first sentence, the bike needs fixing, and, by the way, it’s in on the footpath (‘which is on the footpath’ 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.

That does not add extra information

In the second sentence, there may be other bikes on the property, but it is the bike on the footpath that needs fixing. Sometimes you can delete that without any loss of meaning

The bike on the footpath needs fixing.

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 on the footpath needs fixing.
The bike which is on the footpath 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?’

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