Apostrophes that behave
Apostrophes do exactly two jobs: they show letters are missing, or they show something belongs to someone.
Two jobs, and only two
An apostrophe is never decoration. It is doing one of exactly two jobs, and if it is not doing either of them it should not be there.
Job one: a letter has been taken out and the mark is standing in the gap. Job two: something belongs to somebody.
Count the owners first
For the ownership job, the only question that matters is how many owners there are — and you answer it before you write the mark, not after.
One girl: the girl’s bag. Several girls: the girls’ bags. The mark goes after whatever the owner word already is.
Its and it’s
This is the one that catches everybody, because it breaks the pattern you just learnt.
It’s always means it is. Its, with no mark at all, means belonging to it.
The check takes a second: read the sentence with "it is" in place. If it still makes sense, you want the apostrophe. If it does not, you do not.
Plurals do not take one
More than one of something is just an s. No mark.
Three dogs. Two apples. The 1990s. Writing dog’s when you mean several dogs is the most common apostrophe mistake there is, and it happens because the mark feels like it belongs near an s.
It does not. It belongs near an owner.
Write the owner first, then add the mark
There is an order to this, and doing it the other way round causes nearly every mistake.
First write the word for the owner or owners, exactly as it would look with no apostrophe at all. One dog is dog. Several dogs is dogs.
Only then add the mark. If the word does not already end in s, add ’s. If it does, add just the apostrophe. So dog becomes dog’s, and dogs becomes dogs’.
Names and irregular plurals
A name ending in s follows the same rule. Write the owner first: James. Then add ’s, because James is one person whose name happens to end in s.
Some plurals do not end in s at all — children, men, people. Write the owners first, see there is no s, and add ’s as usual: children’s, men’s, people’s.
That is why "childrens’" is always wrong. There is no such word as childrens to put a mark after.
Worked examples
Which is correct: "the cats bowl" or "the cat’s bowl", if there is one cat?
The bowl belongs to the cat, so this is the ownership job. One owner means the mark goes before the s: the cat’s bowl.
Answer: the cat’s bowl
Three girls share a classroom. How do you write "the classroom belonging to the girls"?
Count the owners: three. The plural is already girls, so the mark goes on the end of it — the girls’ classroom. Nothing is added after the mark.
Answer: the girls’ classroom
Fill the gap: "The bird built ___ nest." (its or it’s)
Try it with "it is": "The bird built it is nest." That is nonsense, so it is not the shortened form. The nest belongs to the bird, so the answer is its, with no mark.
Answer: its
Is there anything wrong with the sign "Fresh apple’s for sale"?
Yes. Nothing belongs to the apple, and no letter is missing, so neither job applies. It is simply more than one apple: "Fresh apples for sale".
Answer: The apostrophe should not be there
Two brothers share a bedroom. How do you write it?
Write the owners first: brothers. It already ends in s, so add only the apostrophe.
Answer: the brothers’ bedroom
Write the toys belonging to the children.
The owners are the children. Children does not end in s, so it takes ’s like any other word that does not.
Answer: the children’s toys
Correct this sentence: "The dog wagged it’s tail."
It’s always means it is, and "it is tail" is not a sentence. The word needed is the possessive its, which takes no apostrophe.
Answer: The dog wagged its tail.
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