Possessed S

According to "The Elements of Style" by William Strunk, Jr. and E.B. White, "Form the possessive singular of nouns by adding 's."

Examples:
Dean's horse
Jennifer's class
the writer's curse

And... if you are reading "Writer's Style Guide: A Complete Guide For Every Occasion" by Mary A. Devries, you will read, "Possession. An apostrophe, with or without an "s", is added to a word to show possession. Notice that only an apostrophe is added when an additional "s" would make the word hard to pronounce."
 
Examples:
Mrs. Weeks' dog
Jesus' disciples
the boys' bathroom
Moses' law
 
Now you know. The apostrophe of a possessive noun belongs in front of the "s" ('s) at most times. After a noun that ends in "s", it should be placed at the end of the word (boss') to make words easier to pronounce. 

However, you can replace difficult sounding words, such as "Ross' son" with "the son of Ross".
Just when you thought the lesson was over, we must remember there are always exceptions! You may wonder where to put the apostrophe on the PROnoun. Guess no more! You don't use the apostrophe.
 
Examples:
its
hers
theirs
yours
ours
Of course, if it is an indefinite pronoun, you should follow the apostrophe rules.
 
Examples:
one's child
somebody else's child

Had enough? Confused and bewildered? Or can you begin to understand? It takes practice, but you will begin to write correctly out of habit. For good measure, I am compelled to leave you with this last tidbit:

Chari and Anne's garden. (Share a garden)
The Hoss's and the William's houses. (Different houses)

When the possessed thing belongs to two or more people (or things), make only the last person (or thing) possessive, unless there is difference in what they possess. 

This is the article's end.