Elementary Rules of Usage

1. Form the possessive singular of nouns by adding 's.
   Follow this rule whatever the final consonant. Thus write,

Charles's friend
Burns's poems
the witch's malice

Exceptions are the possessives of ancient proper names ending in -es and -is, the
possessive Jesus', and such forms as for conscience' sake, for righteousness' sake.
But such forms as Moses' Laws, Isis' temple are
commonly replaced by

the laws of Moses
the temple of Isis

The pronominal possessives hers, its, theirs, yours, and ours have no apostrophe.

Indefinite pronouns, however, use the apostrophe to show possession. one's rights somebody else's umbrella

A common error is to write it's for its, or vice versa.

The first is a contraction, meaning "it is." The second is a possessive.

It's a wise dog that scratches its own fleas.

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Culled from the book ; The Elements of Style by Oliver Strunk