That vs. Which

When it comes to that and which, consistency is absolutely key for making sure you’re conveying the message that you mean to convey. Here’s why they should not be used interchangeably*.

In American English, the general guideline for using that vs. which is: that isn’t always followed by a comma, and which always is. Like so:

The dog that ate the chocolate went to the vet.

The dog, which ate the chocolate, went to the vet.

In such a simple sentence, it might be hard to see why there’s a need to distinguish between the usage of that and which. That’s because here, the only important story is that a dog ate chocolate then went to the vet.

Adding a little more context better conveys just how that and which work.

Of three dogs, one went to the vet. The dog that ate the chocolate — not the dog that sprained its leg nor the dog that swallowed a chicken bone — went to the vet.

As you can see, “that” conveys essential information for identifying exactly which dog went to the vet.

With which:

Some chocolate was found to have disappeared after a dog and cat were left alone with it. The dog, which (happened to have eaten) ate the chocolate, went to the vet.

With the snowy weather, many dogs have been slipping and going to the vet with sprained ankles. This dog, which (happened to have eaten) ate chocolate, also went to the vet.

Many dogs have been going to the vet after eating chocolate. This dog, which ate chocolate too, also went to the vet.

In these cases, a dog that’s eaten chocolate isn’t necessarily the deciding factor for identifying a specific dog that went to the vet, especially in the third sentence where several other dogs have also eaten chocolate. You can see that “which” signals an interesting aside, which could be essential but doesn’t have to be.

Now you can see why that and which should be differentiated, let’s use the two responsibly, shall we?

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*My understanding is that in British English, that and which can be used interchangeably. However, I’d reiterate my former statement that you should be consistent in your usage, whatever function you’d like to designate each, for the sake of maintaining clarity in your writing.