Apostrophes have two uses in ordinary English:
1. To show that letters are left out.
2. To mark a possessive.
The second is actually a special case of the first, as the Editor will explain if you make it all the way to the end of this article.
Apostrophes seem not to be taught very well in our schools. The rules are not very hard, but more writers trip over apostrophes than over any other mark on the page.
So let’s take these two cases one at a time:
1. Letters left out.
Sometimes we see apostrophes used in abbreviations to mark where letters are left out: “Int’l School of Punctuation,” for example, where “Int’l” is shortened from “International.” This style of abbreviation has mostly disappeared in current English.
Contractions are where we most often see the apostrophe for something left out. A contraction is a word made by shortening two words and shoving them together:
It is a beautiful day.
It’s a beautiful day.
You are wrong about that.
You’re wrong about that.
Let us take these two cases one at a time.
Let’s take these two cases one at a time.
I will not be long.
I won’t be long.
As you can see from the last example, sometimes more than just leaving out letters happens when words are contracted. It takes a vowel change, too, to get from “will not” to “won’t.” But the apostrophe’s job is simply to mark where letters have dropped out.
The English possessive case uses an apostrophe and an s to show that something belongs to someone:
This book belongs to Bertha.
This is Bertha’s book.
This proviso belongs to Wilmot.
This is Wilmot’s proviso.
That’s how we do it in the singular, when the owner is just one person or thing. In a plural that already ends in s, we just add an apostrophe:
This house belongs to the Johnsons.
This is the Johnsons’ house.
So far everything is easy. Now come some minor complications.
A plural that doesn’t end in s gets the full apostrophe-s treatment, just like the singular:
This room belongs to the children.
This is the children’s room.
That’s not too hard. But what about when a singular ends in an s, like the name Higgins? Here, the Editor regrets to tell you, anarchy reigns, and there is no set rule.
This car belongs to Higgins.
This is Higgins’s car.
This is Higgins’ car.
Either Higgins’s or Higgins’ is correct, depending on the house style of the publication you’re writing for.
At one extreme, some styles specify that every s or z sound takes only an apostrophe in the possessive:
This crayon belongs to Pertinax.
This is Pertinax’ crayon.
Isaiah was the son of Amoz.
Isaiah was Amoz’ son.
At the other extreme, some styles specify that every singular noun takes an apostrophe and an s:
This cloak belongs to Demosthenes.
This is Demosthenes’s cloak.
There are all sorts of compromises between those two extremes. Learn your house style. If you’re responsible for setting your house style, have fun—just be consistent.
Finally, two rules give people far more trouble with apostrophes than they should, because they’re both flat-out simple rules:
1. There are no apostrophes in possessive pronouns.
This book has lost its cover (not it’s cover).
I think you’ve lost your mind (not you’re mind).
2. There are no apostrophes in plurals.
I found three radios (not radio’s).
This house belongs to the Johnsons (not Johnson’s).
You could stop reading now, but if you’re curious, here’s the explanation we promised at the beginning. It’s a bit of etymology, the science of word histories.
We mentioned earlier that there were two reasons for apostrophes: to show that something is left out, and to mark a possessive. The second is really just a special case of the first. In Middle English, the possessive case was commonly marked by the ending -es or -is. The e or i dropped out, and the apostrophe marks where it used to be. “For Goddes love” in Chaucer is “For God’s love” in modern English; “The Milleres Tale” is “The Miller’s Tale.” Now that you know that, the rule for apostrophes in English is even simpler: Apostrophes mark where letters have dropped out.