William Strunk, Elements of Style (1918)

[7] 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.

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