Grammar for Everyone - Barbara Dykes 2007

Possession
Introduction
Part II The parts of speech

This use causes the greatest confusion, but the rules that apply are, in spite of popular belief, quite straightforward.

Possession expresses the idea that something belongs to someone or some other thing, and omission represented by the apostrophe is actually the contraction of the word ’has’.

For example:

John has money.

becomes

John’s money.

and

The dog has a dish.

becomes

The dog’s (a) dish ...

This is a very economical language device. If the noun is in the plural form, already ending in ’s’, then the use of a second ’s’ would be clumsy. So the apostrophe sits on its own and the second ’s’ is simply omitted.

For example:

The dogs’ dishes ...

The boys’ careers ...

This awkwardness does not apply with plural forms that don’t end with ’s’.

For example:

Children’s teeth ...

In spoken language, to avoid ambiguity, we can fall back on the longer form.

For example:

’The horse’s trainer’ sounds the same as ’the horses’ trainer’, so it is clearer to say ’the trainer of the horses’ or similar.

If a surname ends in ’s’, as in Jones or Fields, the form ’s pronounced es’ (or ’is’) is often used in speech but in writing it is proper to use the apostrophe alone, as with plurals.

For example:

The Jones’ Mercedes