Like the comma, the apostrophe wields amazing power. The talented little dot-and-tail combination (though written at the top of the line, not at the bottom like the comma) can change pronouns to verbs, tell you who owns what, replace a small handful of letters, and make plurals. It comes from the Greek words meaning to turn from or omission.
After commas, apostrophes seem to be the most misused punctuation mark; grammar vigilantes have their work cut out for them.
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Possessive Case of Nouns
With the addition of ’s (or sometimes just the apostrophe), a noun can change from a plain old person, place or thing to a person, place or thing that owns something.
— If a singular noun doesn’t end with an s, add ’s to the end of the noun.
Example: This is Mary and a dog. The dog is Mary’s; Mary is not the dog’s.
— If a singular noun ends with an s, add ’s OR add just the apostrophe to the end of the noun, depending on style guide preference. If the noun is plural and ends with an s, just add an apostrophe.
Examples: Where is Jess’ book bag?
My parents’ house is a lovely old one.
— If you have a compound noun, change only the last one to the possessive.
Example: Mike and Amanda’s new loft apartment is really neat.
— If the possessor is a building, an object, or a piece of furniture, then you don’t need to add an apostrophe to show possession.
Example: The hotel’s room –> the hotel room
Contractions and Omissions
Apostrophes can show an omission of letters, whether as part of a contraction or when showing dialect or accent.
Examples: I’m = I am
Can’t = can not, cannot
Where’s = where is
Apostrophes and Possessive Pronouns
Possessive pronouns don’t require an apostrophe.
Examples: That book is his’. —> That book is his.
No, it’s mine’s! —> No, it’s mine!
The cat washed it’s face. = The cat washed its face. (It’s is a contraction of it is, not a possessive pronoun.)
!! Exception: Indefinite pronouns can be made possessive with an apostrophe.
Indefinite pronouns can be made possessive with the use of ’s. Indefinite pronouns are words like someone, other, and any.
Examples : One – one’s
Is this anyone’s backpack lying here on the ground?
Forming Plurals of Lower- and Upper-case Letters
When you’re trying to describe plural letters, use ’s after the letter so that it’s clearly identified as a letter, not a word. If, for instance, you write is instead of i’s, your reader will get confused between the verb and the letter i.
Example: Mind your p’s and q’s, my dear!