This week, however, is an exception. We want to pick up the question of how to spell the possessive of words that already end in a non-plural (s). (We use .() to show we're talking about the letter - a useful convention from linguistics.) In short, should we call the house of Charles "Charles's house", with (s's), or "Charles' house", with (s')?
Before we deliver our verdict, we should say that we really don't think this is an important question. Most English teachers have far more urgent things to worry about, and in any case how often does the question arise? When did you last have to make this decision in your own writing? And how many marks would you knock off if an otherwise good piece of writing contained what you consider the wrong choice?
Why then are we wasting a whole grammar column - prime time in The TES - on this trivial matter? Because we know that it's often presented as an important part of the Problem of the Apostrophe. When you do apostrophes, this is one of the things you really have to deal with.
Our message is: relax.
Notice that the question here is not when to use an apostrophe. That's a different matter, so let's assume they know it. What we're talking about here is when to write an (s). Is it the Jones's or the Jones', the class's or the class', Illinois's or Illinois', and so on and on? (Just check 'apostrophe' in Google and marvel at the possibilities!)
To our mind, by far the simplest, and therefore best, rule is: if you can hear s or z, write (s). (Once again we follow linguists in writing round items of pronunciation.) This is the rule for ordinary plural possessives like "the children's names". Here you can hear z after "children", so you write (s). In contrast, there's no (s) after the apostrophe in "the boys' names" because there's no separate z.
True, there is a z, but that's the plural ending. The point is that you only hear one z, so you only write one (s).
Now suppose you're a pupil who knows that rule, and along comes the possessive of James. What will you do if you're left alone? You'll apply the rule and write (James's). And will you thank your teacher for suggesting it should be (James')? We don't think so.
Our conclusion is that by far the best approach to this whole question is benign neglect. Who cares if the prince's hat's here and the princess's's there? Keep it as simple as possible: if you can't hear it, don't write it.
There: another trivial problem solved.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk