In contemporary English, apostrophes serve two primary purposes: indicating possession and as a substitute for omitted letters. They can also help when standard conventions for forming plurals are awkward or unavailable and can serve as substitutions for some unavailable punctuation marks.

Common mistakes include:

• Unnecessarily inserting apostrophes to form plurals (“the Smith’s”),
• Adding the letter s to irregular plurals before adding the apostrophe (“mens’ room”),
• Leaving out apostrophes entirely (“mens room”), and
• Using an opening single quotation mark as an apostrophe (“back in the ‘70s”).

In typesetting and word-processing software, the apostrophe is typically set in the directional form (’), otherwise known as a “smart” or “curly” apostrophe. It’s identical to a single closing quotation mark, with the tail curling down and to the left instead of up and to the right. When available characters are limited, such as on some social media platforms, writers are limited to using the apostrophe’s unidirectional form ('), also known as a straight apostrophe.

General Guidelines

Possession

The possessive case is used to show anything from association to physical ownership.

Most singular words form the possessive by adding an apostrophe and the letter s, as with “plaintiff’s” and “respondent’s.” Plurals ending in s or es typically add only the apostrophe, as with “plaintiffs’” and “respondents’.” Plurals that don’t end in s or es typically use the same method singular words do, adding an apostrophe and the letter s, as in “people’s” and “children’s.” “Childrens’,” for example, is incorrect because “children” is already plural.

Words that already have their own possessive inflections, as many pronouns do, don’t require any modification, as with “his,” “her,” “their,” and “its.”

It’s up to the writer whether to add the letter s after a possessive apostrophe for singular words that end in an s or z sound (as with “Texas’s,” “the Jones’s,” and “for convenience’s sake”) and words that end with the letter s but not an s sound (as with “Descartes’s”). Consult your preferred dictionary and usage/style guide and strive for consistency within a given work.

Elision

Contractions, abbreviations, and other omissions of parts of words and phrases typically use apostrophes where letters are omitted, as with “wasn’t” and “Tex. Disciplinary Rules Prof’l Conduct.” In contractions, apostrophes are not typically inserted where space characters are omitted. In abbreviations, one exception to using the apostrophe is when one or more letters are omitted at the end of a word, in which case a period is often used instead, as with the “Tex.” in “Tex. Disciplinary Rules Prof’l Conduct.” This differs from an omission at the end of a word that is not an abbreviation, such as when showing pronunciation in dialogue (“Nothin’ good comes of that”).

When the omission occurs at the beginning of a word or number, as with “the ’70s” or “’Twas the night before Christmas,” be sure to use an apostrophe (’) and not an opening single quotation mark (‘). A trick to get word processors to use an apostrophe at the beginning of a word is to type any letter, then the apostrophe, and then the word or number (as in “b’70s”), and then delete the letter used to trick the word processor.

Plurals

As recently as the ’70s, apostrophes were commonly used to form plurals of numbers (“the 1970’s”), acronyms (“COLA’s”), and initialisms (“ATM’s”). (By their narrower definitions, the term acronym means an all-capital-letter abbreviation pronounced as a word, and the term initialism means one that uses letter-by-letter pronunciation).

Today, though, the trend is toward leaving apostrophes out when forming those plurals, as with “1970s” instead of “1970’s” and “REITs” instead of “REIT’s.” The logic is sound, given that readers are unlikely to consider the lowercase s part of a number or an all-capital-letter abbreviation, and this usage permits the apostrophe to solely indicate possession, allowing a clear distinction between the plurals “1970s” and “REITs,” the singular possessives “1970’s” and “REIT’s,” and the plural possessives “1970s’” and “REITs’.”

When referring to a letter as a letter or a word as a word, the apostrophe is still sometimes used to form the plural. If software allows, the letter or word itself can be italicized, and an unitalicized letter s can indicate the plural, as in “mind your ps and qs.” Not all platforms allow text to be set in italics, though, so apostrophes can clarify between, for example, “as” and “a’s.”

Other uses

Apostrophes can be convenient substitutions for diacritical marks, such as writing the Ó in Irish names as the letter O followed by an apostrophe, and the unidirectional apostrophe (') can be used as a simple substitute for the prime symbol (′) when indicating feet or minutes of arc on a platform that doesn’t offer prime symbols. (Similarly, inches and seconds of arc should use the double prime (ʺ) when available or the unidirectional double quotation mark (") when not available, but never a directional quotation mark (”).)

Placement with other punctuation

Both single and double quotation marks are sentence parts and typically fall outside periods and commas in American English, as in “The code states ‘may,’ not ‘must.’” Apostrophes, prime symbols, and diacritical marks, however, are parts of the words themselves and typically fall inside all sentence punctuation, as in “The property is the plaintiffs’, not the respondents’.”

More-Nuanced Apostrophe Usage

Postpossessive “of”

When to use the apostrophe (“the plaintiff’s real property”) or the postpossessive of (“the opinion of the court”) can be a matter of choice but is sometimes simply tradition. The postpossessive of is rarely used with people, animals, and things that can be held, except with double possessives.

Double possessives

A double possessive results when both the postpossessive of and the possessive form of a word or phrase are used, as in “a client of mine.” Avoid unnecessary double possessives, as in “an heir of the decedent’s.” Try instead either “an heir of the decedent” or “a decedent’s heir.” There is a necessary distinction, however, between “a painting of the decedent” and “a painting of the decedent’s.” To avoid ambiguity, rewrite: “a painting depicting the decedent” versus “a painting owned by the decedent.”

Attributive nouns

An attributive noun is a noun functioning as an adjective, that is, a noun modifying another noun, as in “Veterans Day” and “farmers market.” Whether the attributive noun takes an apostrophe depends on whether the thing being described is for or used by the descriptor rather than owned or organized by it (a day for veterans and a market for farmers). This distinction can get vague, though. For example, a meeting can be both for and organized by shareholders. When in doubt, use the apostrophe (“shareholders’ meeting”) or, to jar the fewest readers, try the postpossessive of (“meeting of the shareholders”).

That apostrophe is not typically used, however, with proper nouns, as with “the Cowboys game” and “the Jones deposition.”

And no, the word mens in “mens room” is not excusable as an attributive noun. The word mens is wrong in any context, because it’s a pluralization of a word that is already plural (men). The nonpossessive attributive form would be “men room,” which is idiomatically appalling, despite “mothers room” seeming just fine.

Separate versus joint possession

Confusion can also reign between separate possession (“plaintiff’s and respondent’s properties”) and joint possession (“plaintiff and respondent’s properties”). For utmost clarity, rewrite: “any property separately owned by either the plaintiff or the respondent” or “any property jointly owned by both the plaintiff and the respondent.” It’s wordier but unambiguous.

Postpositive adjectives

Rewriting can also help when trying to show possession with postpositive adjectives. For example, the singular “attorney general” traditionally forms the plural “attorneys general” (the word attorney is the noun and general the adjective, so attorney gets the s). Instead of writing the plural possessive “attorneys general’s,” try rewriting to avoid the apostrophe.

Proper nouns already containing apostrophes

Careful writing is also necessary to avoid ambiguity with proper nouns already using apostrophes, such as “McDonald’s,” which must function as it is in all roles: singular, plural, singular possessive, and plural possessive.

Possessives modifying gerunds

Nouns modifying gerunds traditionally take the possessive case.

A gerund is the present participle of a verb (ending in ing) functioning as a noun. For example, the word signing in “Upon the parties’ signing this contract, the obligations outlined below . . .” is functioning as the direct object (a noun) of the preposition upon and so is a gerund. On the other hand, the word signing in “The parties signing this contract have an obligation to . . .” is part of a participle phrase and so is not functioning as a noun and is not a gerund.

Whether the ing word is functioning as a noun or participle can be surprisingly subtle. Consider, for example, “the defendants took objection to the witness’s pointing out their unlawfulness” (the objection was to the unlawfulness being pointed out, regardless of by whom, so “pointing out” is a gerund, and witness takes the possessive case) versus “the defendants took objection to the witness pointing out their unlawfulness” (the objection was to who pointed it out, and therefore “pointing out” is a participle, and witness does not take the possessive case).

In those iffy situations, ask yourself whether the ing word is the primary emphasis or whether the noun preceding it is.

Trivia for Word Nerds

Possession: Old English and Middle English used inflection to indicate the genitive case (today’s possessive case), as many other languages still do. The typical inflection in English was an es ending, for example, with the word lordes in the Middle English “in his lordes werre” in Chaucer’s Canterbury Tales. About five centuries ago, English printers added the apostrophe to their inventory of characters for their presses and began to elide the genitive es as an apostrophe and letter s, presumably because by the same time the genitive e had become mostly silent, and “lordes werre” ultimately became “lord’s war.” Thus, even the apostrophe’s possessive use in contemporary English originated with its elision use. Even words that didn’t originally inflect with es for the genitive were eventually adapted to the apostrophe possessive form.

Frequently used words, however, such as personal pronouns, are resistant to change, so those are some of the few words that still inflect to show possession instead of using an apostrophe, as with the words his and her. (Although you can use that knowledge to help differentiate between the possessive its and the contraction it’s, those two words have a long, complicated history of their own.)

Elision: With some words in some dialects, an apostrophe used for the omission of one or more letters can be enunciated with a glottal stop. For example, the colloquial “li’l” might be pronounced either /lil/ or /li?l/, the stop in the latter being like that in the middle of “uh-oh.”

Apostrophes showing elision can be numerous and inconsistent. For example, the spelling of “fo’c’sle” for “forecastle” omits the letter t without substituting an apostrophe (although “fo’c’s’le” is another accepted spelling). The word fo’c’sle also retains its silent final letter e, but gun’l (for “gunwale”) doesn’t. Once again, this is where your preferred dictionary and usage/style guide can help.