I have a job marking students’ papers this fall, and one of the most common errors I see is the misuse of apostrophes in plurals. More often than not, students will insert unneccessary apostrophes. I imagine them saying to themselves, “Does this take an apostrophe? I’m not sure, because no one ever taught me. Oh well, I’ll throw one in. Better safe than sorry.”
In most cases, a simple “s” forms a plural. No apostrophe needed. So we write:
27 cash registers
(I am not sure what this adds up to, but the point is that there are no apostrophes.)
And, contrary to popular belief, we do not use an apostrophe in the plural forms of proper names. So the cute carved and painted wooden signs outside houses that say, “The Smith’s” or “The Cohen’s” are WRONG. Even if the signs were intended to indicate the possessive–i.e. “The Smiths’ House” or “The Cohens’ Cottage”–the apostrophe is still in the wrong place. But that is another topic.
These are the only rare cases where an apostrophe is required along with the “s”:
1. Words used as words:
When you are talking about a certain word, and put it in quotations, the plural is formed with an apostrophe. For example:
Sarah loves to use the word “wrong.” How many “wrong’s” do you think there are on this whole website?
His letter to his lover was full of meaningless “I love you’s.”
If you use italics to indicate a word as a word, you just use a roman (non-italicized) “s” to form the plural:
i love yous
2. Abbreviations with more than one period:
Bob just has one B.A., but Shelly was in school for decades and ended up with two B.A.’s and an M.A. too.
Abbreviations with just one period, like “ed.” for “editor” just have an “s” added before the period, like so: “eds.”
3. Letters used as letters:
An example of a letter used as a letter would be:
The plural is usually formed by adding an “s.”
According to the Chicago Manual of Style, letters used as letters should be written in italics, and the plural is then formed by adding a roman “s.”
But when you use roman type or quotation marks, the plural is formed with an apostrophe, just as in #1 above. This is especially true if there would be confusion without the apostrophe. For example:
There are a lot of “I’s” in your story; it would read more smoothly if you took yourself out of it.
Without the apostrophe, “I” and “s” would spell “is.”