ELEMENTARY RULES OF USAGE
1. Form the possessive singular of nouns with 's.
Follow this rule whatever the final consonant.
Thus write,
Charles's friend
Burns's poems
the witch's malice
This is the usage of the United States Government Printing Office and of the
Oxford University Press.
Exceptions are the possessives of ancient proper names in -es and -is, the
possessive Jesus', and such forms as for conscience' sake, for righteousness' sake.
But such forms as Achilles' heel, Moses' laws, Isis' temple are commonly replaced
by
the heel of Achilles
the laws of Moses
the temple of Isis
The pronominal possessives hers, its, theirs, yours, and oneself have no apostrophe.