Even considering how many ways the apostrophe can be employed, erroneous use of punctuation mark is endemic. Here are brief discussions of ten categories of apostrophe abuse (including one writers and editors must let stand, even though it may pain them to do so).
1. With Plurals
Writing the plural form of a noun in which an apostrophe precedes the plural s, such as when taxi’s is written instead if taxis, is a common error. (This mistake is known as a greengrocer’s apostrophe due to its ubiquity in hand-written—and even printed—store signs.)
2. With Pronouns
Pronouns are followed by an apostrophe and s only as contractions (for example, he’s). Possessive pronouns (such as theirs and yours) never include an apostrophe.
3. With Shared Possession
When two or more people or other entities are described as separately owning something, each name should be in possessive form: “John’s and Jane’s houses are the same color.” But when they share possession, include an apostrophe and an s after the last name only: “John and Jane’s house is just down the block.”
4. With the Possessive Form of a Surname
That shingle on your neighbor’s porch should not read, “The Brown’s house,” unless your neighbor’s legal name is “the Brown.” A sign identifying the residence of the Browns should read “The Browns’ house” (or simply “The Browns”).
5. With the Plural Form of an Abbreviation
No apostrophe is required with plurals of abbreviations. Write, for example, “They disarmed or detonated several IEDs” (not IED’s).
6. With the Plural Form of a Numeral
In the rare case of indicating more than one instance of a numeral, do not use an apostrophe: “Write three 7s on a piece of paper” (not 7’s).
7. With a Span of Years
Some publications persist in using an apostrophe in a reference to a span of years, but that form is outdated: Write, for example, “The style, which flourished briefly in the 1960s, made a comeback several decades later” (not 1960’s) and “He continued to work well into his 70s” (not 70’s).
Generally, an apostrophe should follow a number only if it is possessive (“It was 1985’s longest-reigning Top 40 hit”), though this style is awkward. (An exception is use of a number to stand in for a person, such as when an athlete is identified by a uniform number, as in “It was number 13’s lucky day.”)
8. With the Plural Form of a Word Used as a Word
Don’t apostrophize the conjunctions in “There are no ifs, ands, or buts about it” or the counterpoints in “A helpful list of dos and don’ts follows.” (Do, however, retain the intrinsic apostrophe in the plural form of don’t.)
9. With the Plural Form of a Letter Used as a Letter
Even when a letter is italicized, it still looks awkward to simply place an s next to it to indicate plurality, so do insert an apostrophe: “How many m’s do you spell hmm with?” (Follow this rule even when, in the case of an expression such as “Mind your p’s and q’s,” italicization isn’t necessary.) However, omit an apostrophe when pluralizing capital letters: “She received only As and Bs on her last report card.”
10. With Brand Names
Many brand names, such as Starbucks Coffee, that technically should include apostrophes don’t, for one of two reasons (or both): A company decides that the brand name and/or logo look better without an apostrophe, or it reasons that it’s better to omit the punctuation mark so that people typing the URL for the company’s website into a Web browser or searching for it (or for other references to the company) online won’t have difficulty doing so. Yes, “Starbucks Coffee” is a “mistake,” but one the company has the right to make (and writers and editors have an obligation to honor).