When it comes to possessives — I usually have to think about where to put the apostrophe.
Here’s the rule:
Form the possessive singular of nouns by adding ‘s
Marge’s cool bicycle is clean and spiffy.
The cat’s meow was annoying.
Of course there are exceptions --
When the possessives of ancient proper names ends in -es and -is,
- for conscience’ sake
- for righteousness’ sake
But such forms as Moses’ Laws, Isis’ temple are commonly replaced by
- the laws of Moses
- the temple of Isis
This information was found in The Elements of Style – (Strunk & White)