If you use Microsoft Word with the Grammar Checker turned on, you must know what we're talking about. You type something like this: I like the book which I bought. Then a little wiggly line pops up under book which, linked to the advice that you should consider either adding a comma or changing which to that.
I like the book, which I bought.
I like the book that I bought.
In short, if there's no comma you must use that; which is simply wrong.
How simple, but how wrong. As usual, we rest our case on the best available model of good writing - The TES archives. We found one recent article which defied Bill Gates no fewer than three times (in fact, four times if you include a rather complicated example). It contained phrases like: a positive inclusion partnership which aims to reduce exclusion ...
Pretty good, we think. Ok, as paid hacks of The TES we would, wouldn't we? But we think you'll agree with us.
Mind you, like a lot of misinformation there is a germ of truth in what Mr Gates says. First, comma-free which is much less common in the USA than over here - in fact, we use about four of them to every one used in the States. You may think that shows how much power Microsoft has over there, but these figures were based on news reports from those distant pre-Word days. But even if he's a bit less wrong than we think, he's still not right. There are plenty of good writers even in America who use which without a comma without being noticed by anyone except the little pedant built into every copy of Word.
This whole issue is worth pursuing in a key stage 3 English class because ICT probably means Word, and that means grammar-checkers. The best advice is probably to turn it off and leave it off. Good writers can do it without Bill Gates's (which, incidentally, Word dislikes) help.
But there's a much more important topic for the English lesson: the effect of commas round relative clauses. What is the difference between these two sentences?
His wife who has red hair is called Emma.
His wife, who has red hair, is called Emma.
The commas make all the difference between "defining" and "non-defining" relative clauses - in practical terms, between bigamy and monogamy. The comma which marks a non-defining relative clause really does rule out that: His wife, that has red hair, is called Emma.
The funny thing is that Bill Gates doesn't even notice this one.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk