In grammar, possession is less than nine-tenths of the law

There are many rules in grammar, spelling, and punctuation. It’s important that you follow all of them in order to ensure that your documents are acceptable to all readers. We see many documents in which the authors’ confusion regarding possessive punctuation is evident. The following list, taken from The Chicago Manual of Style (15th edition), will help clear things up:

— Kansas’s legislature
— Chicago’s lakefront
— Burns’s poems
— Marx’s theories
— Berlioz’s works
— Strauss’s Vienna
— Dickens’s novels
— the Lincolns’ marriage
— William’s reputation
— the Williamses’ new house
— Malraux’s masterpiece
— Inez’s diary
— the Martinezes’ daughter
— Josquin des Prez’s motets
— dinner at the Browns’ (that is, at the Browns’ home)
— FDR’s legacy
— 1999’s heaviest snowstorm
— Yahoo!’s chief executive

Exceptions (for names of two or more syllables that end in an eez sound):

— Euripides’ tragedies
— the Ganges’ source
— Xerxes’ armies

and (for words and names ending in unpronounced s):

— Descartes’ three dreams
— the marquis’ mother
— Francois’ efforts to learn English
— Albert Camus’ novels (the s is unpronounced)

but:

— Raoul Camus’s anthology (the s is pronounced)

Other exceptions:

— for righteousness’ sake
— for goodness’ sake
— for Jesus’ sake

but:

— Jesus’s disciples

Source: ProofReadNOW