Roberta, my American highschool English textbook adds a few caveats (that I often forget).
Here's what it says regarding this..
Quote:To form the possessive case of a singular noun, first write the singular spelling of the word. Then add an apostrophe and s. ('s)
woman's shoe
cat's paw
Muir's answer
Burns's poems
Richards's novel
> Exception. The following may be correctly written by adding the apostrophe only: (1) ancient proper names ending in -es, (2) the name Jesus, (3) such expressions as for conscience' sake.
Socrates' pupil
Jesus' birth
Pericles' role
Moses' rod
for righteousness' sake
for goodness' sake
for conscience' sake
To form the possessive case of a plural noun that does not end in s, first write the plural spelling of the word. Then add an apostrophe and s. ('s)
children's books
policemen's protection
To form the possessive case of a plural noun that ends in s, first write the plural spelling of the word. Then add an apostrophe. (')
boy's socks
lions' den
Williamses' house
I was looking through the grammer section of a big English dictionary the other day (maybe it was Oxford's...not sure) & it said the same as above.
I frequently read magazines that use the rules listed above (as in "see pictures of Richards's new baby!" --
apostrophe plus s). Note that that usage is singular. It is referring to a man named Richards, not Richard. If it were "See pictures of the Richards' new baby!", that would be referring to the Richards (as in a family with the family name "Richard"), so that would be plural and only gets an apostrophe (no 2nd s).