|Definition - A punctuation mark (') used to show either possession or omission.|
(1) Bob's cant (shows possession)
(2) Bob can't (shows omission)
1. For plural possession, pluralize the noun first, then add the apostrophe.
two boys' dogs
the Joneses' dog
2. With singular compound nouns, put the 's at the end of the word.
my father-in-law's dog
3. With plural compound nouns, pluralize it first, then add the apostrophe.
my two fathers-in-law's dogs
4. If two people possess the same thing, use 's after the second name.
Bob and Steve's contract will not be renewed.
5. If they don't possess the same thing, use 's after both names.
Bob's and Steve's contracts will not be renewed.
6. Only use it's as a contraction for it is or it has.
It's a beautiful morning.
7. Because they already show possession, don't use apostrophes with possessive pronouns (e.g. his, its, and whose).
The cat hurt its tail.
8. However, use a possessive apostrophe with the word one when it is acting as a possessive pronoun.
a room of one's own
9. Use possessive apostrophes before gerunds.
Bob's partying was getting out of hand.
10. If a pronoun precedes the gerund, use the pronoun's possessive form with no apostrophe.
I hated your inviting me to lunch.
11. Don't use apostrophes to form the plurals for capital letters.
She once dated three MBAs at the same time, but in the end she married a weaver.
12. Don't use apostrophes to form the plurals for numbers.
13. However, use them with capital letters and numbers if needed for clarity.
You must dot your I's.
Etymology - The word apostrophe derives via Middle French and Late Latin from the Greek apostrophos prosoidia, the accent of turning away (from apo, from + strephein, to turn).
Note: The "turned away" aspect of the word's etymology derives from Greek rhetoric where apostrophe denoted either a written passage or a part of an oration that turned away from the subject at hand to address an absent thing or person.
Oxford English Dictionary - Its first citation is from 1588:
"You finde not the apostraphas [? apostrophus], and so misse the accent."
(Shakes. L.L.L. iv. ii. 123)