It’s not always quite as straightforward as adding an apostrophe–s, as I found when I consulted the Oxford Dictionary for Writers and Editors and Hart’s Rules.
The possessive of words, I confess
Is often a cause of great stress.
The great and the wise
“End the word with apostrophe–s”.
If a singular word ends in s,
It always applies – more or less1.
And if it’s a name
Then the rule’s much the same,
Even when there are two, as in “Jess”,
Unless it would sound a right mess –
Then you leave off the s, I would guess2.
And with plurals, it’s clear –
No extra s here.
(Well, that’s sorted out then. Success!)
1. But if the added s would be silent in speech, it’s generally omitted e.g. “for goodness’ sake”.
2. Like “Bridges” (not “Bridges’s”) – also with the possessive of ‘ancient’ names, like Xerxes, Jesus, Herodotus.