Apostrophes are possibly the most misunderstood punctuation mark in English. And yet there is so very little to know about them.

Apostrophes do one job and one job only: they are placeholders for letters which have been omitted from a word. They do nothing more, and nothing less. They were invented by typesetters as a means of saving money, and turned out very handy when writing with a quill as well.

Golden rule: if you can’t figure out which letters have been missed out, don’t use an apostrophe.

That’s it – end of the lesson. Chairs up, go home. Oh, wait. There’s an elephant in the room. What about the possessive apostrophe, do I hear you cry?

It doesn’t exist.

What does exist is a space where letters from an old dead language used to be: and it was those letters which showed that one thing belonged to another. Let me explain.

The old dead language in question is Anglo-Saxon. It was a fully-inflected language and one set of inflections – in this case some letters added to the end of the word – showed that something or someone belonged to something or someone else. That set of letters didn’t survive past Middle English and simply aren’t present in modern English. However, we still need to have a way to show that one thing belongs to another. Modern English does this in two ways: it has a construction that makes use of the word “of” and a very clumsy and limited construction it is. The second way is to cheat by pretending that the ghost of that lost inflection is still alive and well and available for use.

Let’s do some cheating of our own to illustrate this. Let’s use modern English instead of Anglo-Saxon. Suppose I wished to use the cheat’s method of saying “the cow of Mister Brown”. I’m going to resurrect the lost inflection and add it into a rearranged version of the sentence:

Mister Brownes cow. (added possessive case inflection –es)

Hold that thought. Let’s say we have one cow owned by two families, both called Brown. Logically, then we have to refer to the plural Browns. Let’s connect the cow to the Browns by using our inflection:

The Brownses cow. (added plural –s, added possessive case inflection –es)

I’m sure you’re way ahead of me now. In the first case, the letter which, for no good reason that I can think of, got dropped out was the “e”. Put in the apostrophe as a placeholder and we have

Mister Brown’s cow. So far, so good – it’s what we expected.

Apply the same logic to the second case and we arrive at

The Browns’s cow. Oh, dear – unexpected result.

English speakers and writers are extremely good at looking at logical structures, finding them too hard, and simplifying them. Some simpleton of an accounting disposition looked at this example and said “we can be even more cost-efficient!” and figured out that the extra “s” hanging around on the end could have been dropped out with its “e”: and so we arrive at

The Browns’ cow. Just what we expected.

So now we have the explanation as to why that apostrophe moves from one side of the terminal “s” to the other: it’s because that terminal “s” doing a different job in each case, but in both cases the apostrophe is doing what apostrophes do, which is showing where letters have been missed out.

Possessive pronouns like his, hers, its, ours, yours, theirs don’t have any letters missing, so they don’t have apostrophes, either.

As an aside: I worked this out by myself about 30 years ago and thought I was very clever indeed because it seemed that nobody else knew about it and relied on learning a “possessive rule”. Not so long ago, I heard a radio interview with an Australian writer who had just put together a book on grammar and punctuation and the topic of the interview was – you guessed it – the origin of the so-called possessive apostrophe, pretty much as I’ve explained it here. On the one hand, deflating, but on the other hand it’s nice to know that I haven’t misled anybody, either.

I simplified the Anglo-Saxon and Middle English grammars so as not to spoil the story.