To make possesives: add 's to singular nouns except Classical and Biblical names ending in s; add only ' to plural nouns except plurals which do not end in s.

Examples: dog > dog's, Ms. Jones > Ms. Jones's, Augustus > Augustus', Jesus > Jesus', dogs > dogs', children > children's, mice > mice's

Note the above rule does not apply to pronouns: it > its, not it's.

There are a couple of minor complications, but the above rule will hold good 999 times out of a thousand.

Bingley

Bingley