Apostrophe Errors That Survive Spell Check

A supervisor reading a thesis draft circles two words on the same page, its and it's, both spelled correctly, both real English words. Neither one is wrong on its own. Only one of them is right in that sentence, and a spell checker has nothing to say about which.

Apostrophes do exactly two jobs in English: they mark a noun as possessive, and they mark missing letters in a contraction. Apostrophe rules for possessives and contractions exist for the cases where those two jobs get confused, or where a mark appears that neither job actually calls for. This sits inside the wider set of academic writing mechanics a manuscript gets checked against, and apostrophes are the pattern most likely to survive every automated check untouched.

Apostrophe Rules for Possessives and Contractions: Where Spell Check Fails

Its is the possessive, its title page, and it's is the contraction, it's time to submit. A spell checker works by dictionary lookup: it flags a string of letters that matches nothing in its word list.

The test that holds in every case is to expand it's to it is or it has and read the sentence back. If the expansion still makes sense, the apostrophe belongs. If the sentence describes something a noun owns, there is no apostrophe at all, the same reason his and her carry none. A thesis that gets this wrong once is a typo. A thesis that gets it wrong five times in one chapter reads as a document nobody proofread a second time.

Possessives of Singular Nouns Ending in S

A single noun, which already ends with an s, will take a possessive apostrophe. Most modern style guides will also need a more s. So for example: the boss's decision, Charles's letters, the class's results. This is the one apostrophe rule where careful writers genuinely disagree. The disagreement is old, not a sign either side is careless.

Wiley-Blackwell's own house style guide for its academic journals states the rule plainly: a word ending in s should still be followed by an apostrophe and an s, giving Claudius's reign rather than Claudius'. A smaller, older convention drops the extra s for classical and biblical names that already end in an unstressed s sound, so Xerxes' army and Achilles' heel keep the bare apostrophe even in a document that adds 's everywhere else.

Institutions do not always resolve the question themselves. The same house style guide notes St Thomas' Hospital, apostrophe alone, next to The Queen's College, Oxford, apostrophe plus s, as two real institutional names that made opposite choices. The working rule is to pick one convention for a document and hold it, and to leave a fixed proper name exactly as it is actually written rather than correcting it to match a personal house style.

Convention	Why	Example
Add 's to every singular noun, even ones already ending in s	the standard modern rule, used by Wiley-Blackwell's academic house style among others	Claudius's reign, the boss's decision
Add only the apostrophe, no extra s	an older convention kept for classical and biblical names ending in an unstressed s sound	Achilles' heel, Xerxes' army
Leave a fixed proper name exactly as it is written	applies to institutions, brands and titles rather than ordinary nouns	St Thomas' Hospital, The Queen's College, Oxford

Plural Possessives: One Owner or Many

The apostrophe in a plural possessive goes on the other side of the s that makes the noun plural. Who owns what is different because of the position of one character. The students' papers means the papers belonging to more than one student. The student's paper means one student, one paper. Both are grammatically valid, spelled with the same letters, with only the apostrophe's position telling them apart.

A noun with an irregular plural, one that does not simply add an s, keeps the ordinary rule and adds 's to whatever the plural form already is: the children's books, not the childrens' books, because children is already plural without an s for the apostrophe to sit after. The same logic covers people's choices and women's research. Checking which word is actually plural before placing the apostrophe catches nearly every error in this category.

Joint Possession: One Apostrophe or Two

Two names sharing one thing take a single apostrophe on the second name only: Smith and Jones's 2019 study means one paper the two of them wrote together. Two names owning separate things each take their own apostrophe: Smith's and Jones's 2019 studies means two different papers, one from each author.

The distinction rarely changes a sentence's grammar, only its meaning, which is exactly why it is easy to miss on a read-through. A literature review citing several co-authored papers is where this shows up most, and the fix is to ask, before placing the apostrophe, whether one thing is being shared or several things are being kept separate.

Where the Apostrophe Never Belongs

A decade, an acronym or a number does not take an apostrophe when it is made plural, because none of the three is a possessive and none of them is missing any letters. The 1990s, not the 1990's. PDFs, not PDF's. Three PhDs graduated that year, not three PhD's.

The error is common because a plural ending in s can look, at a glance, like it needs the same treatment as a possessive ending in s, and a writer reaching for a plural sometimes reaches for the wrong mark out of habit rather than reasoning it through. A free punctuation checker catches this kind of mismatch automatically, which matters most exactly when a slow, careful read cannot be guaranteed before a deadline.

TextPulse's free tools collection covers the rest of what a spell checker misses too, grammar, formality and spelling convention, for a single pass that goes beyond any one mark.

The grammar mistakes that recur most often in research papers are collected separately, and the spelling question underneath many of them, US against UK conventions, has its own page.

None of these five patterns needs memorising in the abstract. What they need is a slower read of exactly the sentences a spell checker already marked clean, since that is precisely where an apostrophe error survives. A supervisor who circles its and it's on the same page rarely stops reading there, and the next thing they check is whether the argument gets the same careless treatment as the punctuation around it.