Apostrophes
The apostrophe does only two jobs in English: it marks possession and it marks the missing letters in a contraction. Everything else — the plural s on 'banana's' at the fruit stand, the random mark dropped into 'the 1990's' — is a mistake, and it's the single most common punctuation error native speakers make, precisely because the mark is asked to carry two unrelated jobs on one small symbol.
Possession: 's for the vast majority of nouns
Add 's to a singular noun to show possession, no matter how the word ends: the dog's leash, the boss's office, Charles's book. Style guides split on names already ending in s — Chicago and most newspapers add 's (Charles's), while some (notably certain British house styles and the AP for classical/biblical names) drop the s and keep only the apostrophe (Jesus', Achilles'). Pick one convention per document and stay consistent; both are defensible, so the error is inconsistency, not either choice alone. For a plural noun that already ends in s, add only the apostrophe after it: the dogs' leashes, the Smiths' house, three weeks' notice. This is where the greengrocer's apostrophe usually starts — someone assumes any word ending in s that's followed by a noun needs an apostrophe before the s, and 'the dogs's leash' or worse, a stray apostrophe on a plain plural like 'apple's for sale', creeps in.
Irregular plurals keep the 's
Nouns with irregular plurals — children, men, women, people, mice — take 's just like singulars, because they don't already end in s: the children's playground, the women's conference, the mice's nest. There's no exception here; the rule tracks the spelling of the plural, not some deeper logic about the group being 'plural already.'
Joint vs. separate possession
When two people own one thing together, only the second name takes the apostrophe: Sam and Lee's apartment (one shared apartment). When each owns a separate thing, both names take it: Sam's and Lee's apartments (two different apartments). This distinction rarely gets tested in casual writing, but it matters in legal and real-estate prose where the difference between joint and several ownership is the whole point of the sentence.
Contractions: the apostrophe stands in for missing letters
In a contraction, the apostrophe sits exactly where the omitted letters used to be: do not becomes don't (the o of 'not' is dropped), it is becomes it's, they are becomes they're, cannot becomes can't (an irregular contraction — the whole word compresses, not just a letter drop). Because the apostrophe marks an omission, its placement isn't arbitrary: 'would not've' for 'would not have' is nonstandard exactly because it stacks two contractions sloppily, though it shows up in informal transcription of speech.
The one that trips up almost everyone: it's vs. its
This deserves its own callout because it accounts for a disproportionate share of apostrophe errors. It's is always and only the contraction of 'it is' or 'it has': it's raining, it's been a long day. Its is the possessive form of it, and — like his, hers, its sibling possessive pronouns — it never takes an apostrophe: the cat licked its paw, the company revised its policy. English possessive pronouns (his, hers, its, ours, theirs, yours) are the one place possession is NOT marked with 's, which is precisely why its keeps getting apostrophized by analogy with regular nouns. If you can substitute 'it is' or 'it has' into the sentence and it still makes sense, you want it's; if not, you want its.
Never use an apostrophe for a plain plural
This is the 'greengrocer's apostrophe,' named for handwritten market signs like 'Banana's — 50p' and 'CD's for sale.' Plain plurals of nouns never take an apostrophe: bananas, CDs, 1990s, the McDonalds next door (referring to more than one member of the family). The confusion partly comes from a genuine, narrower exception: some style guides do use an apostrophe when pluralizing a single lowercase letter to avoid ambiguity — mind your p's and q's — because 'ps and qs' reads oddly. Even that exception is shrinking; Chicago now recommends 'Xs and Ys' or italics for the letter instead of an apostrophe wherever clarity allows it, reserving the apostrophe-plural only for genuinely ambiguous single lowercase letters.
Decades, acronyms, and abbreviations
Decades don't need an apostrophe before the s: the 1980s, the '80s (note the apostrophe here replaces the dropped '19,' not marking a plural — it's a contraction-style apostrophe, and it curls the correct direction, like a closing quotation mark, not like an opening one, a detail most word processors get backwards by default). Acronyms and initialisms also pluralize without an apostrophe: DVDs, ATMs, CEOs. The apostrophe-s pattern (CD's, ATM's) is never correct for a plural, only ever for a possessive (the CD's case was cracked) or, rarely, to avoid misreading.
Compound nouns and possession
For compound nouns and titles made of several words, the apostrophe goes at the very end of the whole phrase, not on the head noun in the middle: my mother-in-law's advice, the attorney general's decision, someone else's problem. Because English speakers instinctively look for the 'head' noun to mark it (mother, attorney, someone), 'my mother's-in-law' and similar mid-phrase apostrophes are a recurring error — the possessive marker attaches to the entire noun phrase as a unit, not to whichever word inside it feels most noun-like.
Apostrophes with numbers and symbols
When pluralizing numerals or symbols, most current style guides skip the apostrophe entirely: the 1900s, size 7s, three A's on the exam is more traditionally styled as three As now in most style guides, though 'A's' persists informally to avoid the odd look of 'As' next to a real word. The general trend across the last several decades of style-guide revisions has been to strip the apostrophe out of more and more plural contexts, leaving it almost exclusively for possession and contraction — which is worth remembering as the long-term direction the rule is moving, even in cases still marked as acceptable exceptions today.
A brief history of why the apostrophe is so inconsistent
The apostrophe entered English typography in the 16th century, borrowed from French printers, originally just to mark omitted letters. Its use for possession came later and by analogy — 'his book' contracted in some dialects to 'hisbook' and then, confusingly, some grammarians mistakenly believed the possessive 's on nouns (the king's crown) was itself a contraction of 'his' (the king his crown), so they extended the same omission-mark apostrophe to possessives too. That folk etymology was wrong — the possessive -s actually descends from an Old English genitive case ending, nothing to do with 'his' — but the apostrophe convention it produced stuck, which is part of why the mark still feels like it's doing two unrelated jobs: historically, English grammarians grafted a punctuation habit from one use onto an entirely separate one, and neither modern usage nor most speakers' intuitions ever fully reconciled the two.
Apostrophes in dialect and quoted speech
Writers sometimes use apostrophes to mark dropped letters in represented speech or dialect — 'goin'' for 'going,' ''tis' for 'it is,' 'ol'' for 'old.' This is the same omission logic as a standard contraction, just applied more freely and informally; the risk is overdoing it to the point that dialogue becomes hard to read, which is why most editors recommend using this technique sparingly and relying on word choice and syntax, rather than heavy apostrophe-marked spelling, to convey a character's voice.
A quick self-check before you type one
If you're about to type an apostrophe, ask which of the two legitimate jobs it's doing: showing that something belongs to something else, or standing in for letters you've dropped in a contraction. If neither applies — if the word is simply plural, or a possessive pronoun like its, hers, or theirs — the apostrophe doesn't belong there at all, no matter how natural it might look on the page.
Frequently asked questions
Does 'the 1990's music' need an apostrophe?
No — as a plain plural referring to the decade, it's 'the 1990s.' You'd only add an apostrophe if the decade possessed something in a way that required the possessive form, which almost never happens in practice ('the 1990s' sound, not 'the 1990's sound').
Is it 'James' car' or 'James's car'?
Both appear in edited prose. 'James's car' (adding 's) is the more common recommendation from Chicago, APA, and most American style guides; 'James' car' (apostrophe only) is common in British newswriting and for certain classical or biblical names. Choose one rule and apply it consistently within a single document.
Why doesn't 'its' have an apostrophe like other possessives?
Because English possessive pronouns — his, hers, its, ours, yours, theirs — form their own closed class and never take an apostrophe, unlike possessive nouns (the dog's, Maria's). It's an inherited quirk of how pronouns evolved separately from nouns in English, not an inconsistency someone forgot to fix.
Do plural family names ever take an apostrophe on a sign like 'The Smith's'?
Only if the sign means something belongs to the Smiths (The Smiths' house). If the sign just names the household, the correct plain plural is 'The Smiths,' no apostrophe at all — a mistake so common on mailboxes and holiday cards that some style writers call it the 'family sign apostrophe.'