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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.