If you are not sure about when to use the word ‘which’ then you are in good company. I see the word ‘which’ in the wrong place almost every day. Of all the troublesome pieces of grammar – and there sure are a lot – the incorrect uses of ‘which’ and ‘that’ seem to be one of the stickiest.

Does it matter? That depends on the impression you want to create. If mastery, integrity and attention to detail are among your brand values, the answer is yes, it does matter. All grammar matters, as I mentioned in last week’s blog, ‘10 things I learned from 15 years at BRW’.

‘Which’ is interchangeable with ‘that’ in one circumstance:

She passed the pepper grinder that was broken.

This means, she passed the broken pepper grinder.

It is now acceptable (but in my view undesirable) to say:

She passed the pepper grinder which was broken.

This is known as a restrictive relative clause – in other words, the clause (that was broken) describes the pepper grinder. It’s not just any pepper grinder; it’s the broken one.

So, why be pernickety about the use of ‘which’ in this situation? Because the simple addition of a comma – also widely misunderstood and misused – can make the sentence completely different, and potentially wrong.

She passed the pepper grinder, which was broken.

You are now reading a nonrestrictive relative clause. That means the clause could be left out without affecting the meaning or the structure of the sentence. The most important information is that she passed the pepper grinder, regardless of whether or not it was broken.

Here’s a real life example from a very informative and excellent website:

Embryos which are not transferred immediately back to the mother during the initial IVF cycle are placed in cryo storage, through a process called verification, for use in later cycles.

It’s not wrong, but it would be unambiguous if it said, “Embryos that are not transferred …” especially as the use of the comma after cryo storage triggers us to think we have just read a nonrestrictive relative clause. (I would swap the commas for em dashes, Rog.)

The word ‘that’ can never be used to introduce a nonrestrictive relative clause.

While we are on the subject of ‘that’, remember not to mix it up with ‘who’. For example”

The company who bought the shipment of gold is now selling it cheap. Wrong. ‘Who’ describes a person.

The company that bought the shipment of gold is now selling it cheap. Correct.

If you don’t believe me, of course, you can always go to the source. I recommend the Oxford English Dictionary for those curly moments of grammar that every content marketer will face.