Pop quiz – spot the error in the following sentence:

“IT security is especially important for companies which have seen large numbers of employees suddenly working from home.”

If you spotted the word which and thought, “It should be that,” give yourself a shiny gold star. Because you’re right: it should be that. In other words, it should be:

“IT security is especially important for companies that have seen large numbers of employees suddenly working from home.”

Why? I could go into a by-the-book explanation involving relative pronouns, restrictive clauses and non-restrictive clauses but there’s a simpler way to put it: use which for descriptions the sentence can do without and that for descriptions that are essential.

Let’s use a couple of examples:

“In today’s business climate, which is more volatile than ever, many CEOs emphasise the importance of being agile and flexible.”

“In a business climate that is more volatile than ever, many CEOs emphasise the importance of being agile and flexible.”

In the first sentence, you can take out that entire middle phrase – “which is more volatile than ever” – and the rest still forms a complete and coherent thought: “In today’s business climate, many CEOs emphasise the importance of being agile and flexible.” In other words, the middle phrase is not essential to the sentence, so it needs to use which.

In the second sentence, there’s no way to remove the “that is more volatile than ever” bit and have the rest of the sentence make sense. It’s an essential description, so it requires that.

Here’s another example, taken from Random House copy chief Benjamin Dreyer’s book, ‘Dreyer’s English: An Utterly Correct Guide to Clarity and Style’:

If you’re about to offer a piece of information that’s crucial to your sentence, offer it up without a comma and with a “that”:

Please fetch me the Bible that’s on the table.

Which is to say: Fetch me the Bible that is on the table rather than the Bible that’s under the couch or the Bible that’s poised picturesquely on the window seat.

If you’re offering a piece of information that’s perhaps interesting amplification but might well be deleted without harm, offer it up with a comma and a “which”:

Please fetch me the Bible, which is on the table.

One Bible and one Bible only.

Need a little trick to remember which sentences need which and which need that? ‘Grammar Girl’ Mignon Fogarty suggests an easy little mnemonic device:

If you think of the Wicked Witch (Which) of the West from The Wizard of Oz, you know it’s OK to throw her out. She’s bad, so we want to get rid of her. We’re going to throw out the wicked witch, just like you can throw out the ‘which’ part of your sentence. You won’t change the meaning of the sentence without the ‘which’ phrase. So, you can throw out the which (or witch) phrase, commas and all.

Those whiches you can’t throw out? Use a bit of editing magic to turn them into thats instead.

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