Well, after weeks of not being able to find the existing copy of Strunk and White's Elements of Style, I went to Barnes and Noble and got a new one! It is with great pleasure that I introduce to you the first part of the guide: ELEMENTARY RULES OF USAGE.
Ready? On to today's rule:
"1. Form the possessive singular of nouns by adding 's
Follow this rule whatever the final consonant. Thus write,
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 possessive hers, its, theirs, yours and ours have no apostrophe to show possession.
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."