A Tip for the Possessive Apostrophe
The following rules apply:
1. If the possessive noun is singular, always add an apostrophe + s.
2. If the possessive noun does not end in -s (in its written form), always add an apostrophe + s.
3. If the possessive noun is plural and ends in -s (and this is a characteristic feature of the large majority of plural nouns), just add an apostrophe.
4. If the possessive noun is singular, always add an apostrophe + s.
5. If the possessive noun does not end in -s (in its written form), always add an apostrophe + s.
6. If the possessive noun is plural and ends in -s (and this is a characteristic feature of the large majority of plural nouns), just add an apostrophe.
This is why you say James's books, the children's books, and even the boss's books. This is also why you don't know whether something belongs to one or several boys until you see the sentence in writing.
To simplify things further, all you really have to remember is Rule 3. If Rule 3 does not apply, always add an apostrophe + s.
(Note that American English does not always follow these rules.)
Source: TEFL.com