For such a little mark, there are a lot of rules for the apostrophe, so let’s get right to it.
1. Use the apostrophe with contractions. The apostrophe is always placed at the spot where the letter(s) has been removed.
Examples: I don’t (do not) understand. That isn’t (is not) what I meant.
2. Use the apostrophe where the noun that should follow is implied.
Example: This was her mother’s, not her, book.
3. Use the apostrophe to show possession. Place the apostrophe before the s to show singular possession.
Examples: one dog’s bone; one woman’s car; Peter’s house
NOTE: Although names ending in s or an s sound are not required to have the second s added in possessive form, it is preferred.
Boris’s paintings; Kansas’s highways; Mr. Juarez’s money; Mr. Jones’s books
4. To show plural possession, make the noun plural first. Then immediately use the apostrophe.
Examples: two dogs' bones; the Juarezes’ money; the Joneses’ books
5. With a singular compound noun, show possession with 's at the end of the word.
Example: my mother-in-law's hat; my sister-in-law’s bike
6. If the compound noun is plural, form the plural first and then use the apostrophe.
Example: At the reunion my sisters-in-law's kids ran wild.
7. Use the apostrophe and s after the second name only if two people possess the same item.
Paul and Tracy’s party was awesome. (joint ownership of the party)
Paul’s and Tracy’s parties were awesome. (separate ownership of the parties)
Paul and Tracy’s parties will be held on different days. (joint ownership of more than one party)
8. Use the possessive case in front of a gerund (-ing word).
Examples: Jayne’s diving was perfect.
A notebook would help Beverley’s critiquing of the restaurant.
9. The plurals for capital letters and numbers used as nouns are not formed with apostrophes.
Examples: John spoke with three M.D.s. (no apostrophe needed)
John went to three M.D.s' offices. (the apostrophe is needed here to show plural possessive.)
the 1990s not the 1990's; the '90s or the mid-'70s not the '90's or the mid-'70's
She learned her times tables for 6s and 7s.
Exception: Use apostrophes with capital letters and numbers when the meaning would be unclear otherwise.
Examples: Please dot your i's. (Without the apostrophe you would have the word is.)
Ted couldn't distinguish between his 6's and 0's. (You need to use the apostrophe to indicate the plural of zero or it will look like the word Os. To be consistent within a sentence, you would also use the apostrophe to indicate the plural of 6's.)
10. The only time an apostrophe is used for it's is when it is a contraction for it is or it has.
Examples: It's a good cup of tea. (It is a good cup of tea)
It’s been great talking with you. (It has bee good talking with you.)
11. Do not use an apostrophe for the plural of a name.
Examples: I saw the Juarezes in Toronto. NOT I saw the Juarez’s in Toronto.
The Joneses live in the suburbs. NOT The Jones’s live in the suburbs.
12. Never use an apostrophe with possessive pronouns: his, hers, its, theirs, ours, yours, whose. They already show possession so they do not require an apostrophe.
Correct: This book is hers, not yours.
Incorrect: Sincerely your's.
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