From Word Genius
The apostrophe has been around for centuries. It first emerged as a punctuation mark over 500 years ago in Western Europe, and its usage has confused people ever since. Prominent writers including Jane Austen, Thomas Jefferson, and Benjamin Franklin were known to be inconsistent with their apostrophe usage, and even Shakespeare himself had trouble with this pesky punctuation mark. Today, apostrophes are used in English for three main reasons: to show possession, to omit letters or numbers, and to pluralize singular letters or numbers.
1 – To Show Possession
Apostrophes are the easiest way to show possession. Instead of saying, “That is the book of Sarah,” it’s much simpler to say, “That is Sarah’s book.” However, there are many nuances to using apostrophes for possession.
Showing Possession With Nouns
With all singular nouns: Add an apostrophe and the letter “s” to the end of the word (even if it ends in an “s”).
Examples: the bird’s nest, the witness’s testimony, Boston’s weather
Note: These guidelines depend on the chosen style guide. Most (AP, APA, MLA, and CMOS) advise adding an “-’s” when a singular common noun ends in an “s,” but individual publications or internal style guides might call for something else. For example, AP doesn’t add the “s” to singular proper nouns. It would be “the bird’s nest,” but “Chris’ car.”
With plural nouns that do NOT end in “s”: Add “-’s” to the end of the word.
Examples: the children’s books, the sheep’s wool, the men’s clothing
With plural nouns that DO end in “s”: Add only an apostrophe.
Examples: my friends’ jobs, two weeks’ time, the Smiths’ new car
With nouns that end in “s” and are the same in singular and plural form: Add only an apostrophe.
Examples: the scissors’ blades, the species’ habitats
With two or more nouns to show joint possession: Add “-’s” to the last noun listed.
Examples: Mark and Andy’s house, mom and dad’s vacation
With hyphenated nouns: Add “-’s” to the end of the word.
Example: my mother-in-law’s ring
Showing Possession With Pronouns
With singular AND plural possessive pronouns: Do NOT use an apostrophe with possessive pronouns such as “his,” “hers,” “its,” “theirs,” “ours,” and “yours” (they imply possession).
Example: That cooler is theirs.
With indefinite pronouns: Add an “-’s” to indefinite pronouns such as “anyone,” “anybody,” “everyone,” “nobody,” and “someone” to indicate possession.
Examples: someone’s book, one’s attitude, nobody’s business, each other’s faces (The latter is a common mistake. “Each” is always singular, so the apostrophe would never be on “each’s,” for example.)
With interrogative pronouns: Do NOT use an apostrophe.
Example: Whose paper is this?
2 – To Omit Letters or Numbers
Apostrophes are handy tools to shorten words or numbers, especially in casual conversation and writing.
Use an apostrophe to create contractions by omitting certain letters, thus combining two words into one.
Examples: are not = aren’t, let us = let’s, I would = I’d, you have = you’ve
Use an apostrophe to shorten numbers. Time periods, such as “the 1980s,” do not need apostrophes for pluralization — only for shortening, as in “the ’80s.” The exception is a possessive form of a specific year, as in, “The firm closed during 1929’s stock market crash.”
Examples: the class of 2012 = the class of ’12, the 1960s = the ’60s
Use an apostrophe to shorten the beginning or end of other words in informal writing or speech.
Examples: it is = ’tis, fishing = fishin’, rock and roll = rock ’n’ roll, because = ’cause
3 – To Form Plurals of Letters or Numbers
The only time that an apostrophe should be used to pluralize something is for a singular letter or a number. However, some style guides, such as APA, advise against using an apostrophe this way.
Use an “-’s” to indicate the plural form of a singular letter or a number.
Examples: I got straight A’s last semester” (Using the term “As” is incorrect, because “as” is already a word)
She received two 9’s and one 10 for her dance routine
Using apostrophes to form plurals is also common in a lot of colloquial idioms and phrases.
Examples: “Dot your i’s and cross your t’s” (Pay attention to detail)
“X’s and O’s” (Abbreviation for “kisses and hugs”)
“Minding your p’s and q’s” (Be careful; avoid mistakes)
“The do’s and don’ts” (This common phrase is written differently depending on what style guide is used. AP and others recommend “do’s and don’ts” to pluralize the words, while CMOS says “dos and don’ts” is correct.)
Do NOT use an apostrophe to pluralize capitalized abbreviations. Simply add an “s.”
Examples: CDs, MP3s, DVDs, RNs
When using an apostrophe alongside other punctuation, the other punctuation mark(s) should come after the apostrophe. This is true for any punctuation, including commas in the middle of a sentence. (Note: This applies when the mark is used as an apostrophe, not when it is used as a single quotation mark.)
Examples: Jump in, let’s go swimmin’!
The award was the Williams’, but they left before the ceremony.
In quotation marks: The punctuation and quotation marks come after the apostrophe.
Example: One of Tom Petty’s greatest hits was “Free Fallin’.”
At the end of double quotation marks: The punctuation and all quotation marks come after the apostrophe. (Note: The single quotation mark looks similar to the apostrophe, but it does not function as an apostrophe and the usage rules are different.)
Example: She explained, “Tom Petty is the best—my favorite song is ‘Free Fallin’.’”
Obscure proper nouns: Some proper nouns omit an apostrophe without rhyme or reason. If you live near one of these places, or if you have another reason to use them, just memorize them.
Examples: Kings Mills, Pikes Peak