That, which or who? This is a set of words that I see used incorrectly all the time, especially using “that” instead of “who” (although there are a few debates, it’s normally quite clear). I’ve also been asked for help on that/which a number of times, and I have to admit that I wouldn’t have been able to reel off the rules without checking it. Of course, I do check all of these, even when I think I know the answer, just to make sure I’m giving you the correct information!

So, to start off, you can use that OR which if you are introducing clauses that define or identify something (the fancy name for these is “restrictive relative clauses”) and it doesn’t seem to matter which – it’s a question of style preferences or what feels better in the sentence (wouldn’t you know: another one without a proper rule!) So: “A book which aims to explain all human life”, “a book that aims to explain all human life”.

Which is officially used (instead of that) if the clause gives additional information. “The book, which costs £15, has sold 1000 copies”.

Although it’s not officially specified in my reference books, I would therefore use them like this:

– If you’re just saying what the book (or whatever) does in general, use that: “these are the books that will tell you about the stars”.

– If you’re explaining something in comparison with something else, use which: “This is the book which explains all human life, unlike this other one, which just explains about men”.  The way to remember this? “Which is which?”

Moving on to who, we use who when we’re talking about a person or something that’s personified such as a group of people or a named animal. “The man who said yes”, “The proofreaders, who were all a bit pernickity”, “Felix the cat, who was very naughty” (and possibly, “the cat, who was very naughty”, if it’s a specific cat, but “the cats that lived in the barn”, “the cat that I saw on my way to work, which was white with a grey tail … “).

Things do get a bit confusing when you get to a group of people, as a group is non-personified, but the people are – you can do it either way but someone will argue with you, whichever path you take (“The group of men who were going to the ball”, “The group of men that was going to the ball” – I prefer the former, personally. Remember to make the verb agree when you do this – it depends whether you’re referring to the singular group or the plural members of the group).

You can find more troublesome pairs here and the index to them all so far is here.