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, the
Beatles’s concert tour
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.
Source: The Elements of Style by William Strunk, Jr.
