1. Plural nouns not ending in 's' take both the apostrophe and the 's':
"The children's toys."
"The men's cars."

2. Nouns that take -s for the plural add only the apostrophe to indicate possession (this is called 'zero genitive'):
"The boys' toys."
"The dogs' tails."

3. The 'zero form' is also used:
a. with Greek names of more than one syllable:
"Socrates' wife."
"Euripides' plays."

b. with nouns ending in the sound /z/, in writing only the apostrophe is often used, butthe usual pronunciation seems to be /iz/, as if the 's' were actually there. But there are inconsistencies, and both forms seem to be accepted.
You usually write: "Burns' poems" and "Dickens' plays".
The most common pronunciation, however, seems to be /'b3nziz/ and /'dikinziz/.
(The "i" in these words is supposed to be the English 'short i' sound)

c. In fixed expressions with 'for... sake', the zero form is also used.
"For goodness' sake."

4. All other names ending in a sibilant sound other than /z/ (see 3.b.) have the regular 's genitive:
"Ross's theories."
"Tess's new dress."

A bit messy, isn't it?

Miriam

I agree with Boston . Its hard to follow a response of an expert who uses too much difficult words than who better prefer to be simple in writting.