I fielded a question recently about "minding your p's and q's." I had used that expression in a column a few weeks ago, and it prompted an email from a reader named Christine who was curious about the apostrophes.

Anyone who knows anything about apostrophes knows that using them to form plurals causes some of worst writing errors a person can make: Eat your carrot's. There are two Julie's in my class. We're playing tennis with the Smith's this weekend. She goes to a lot of spa's.

Those are all mistakes because, in most cases, apostrophes don't form plurals. But the key phrase here is "in most cases." When you learn your way around apostrophe rules, you see why "eat your carrot's" is a terrible mistake but "mind your p's and q's" is not.

The apostrophe has two main jobs. It forms possessives and it indicates omitted letters or numbers, usually in what we call contractions.

"That is Suzanne's house." In this sentence, you add an apostrophe and an S to Suzanne to show that she owns the house. She possesses it.

It's not always that simple. There are different rules for forming possessives of plural nouns.

For most plurals, you form the possessive by putting an apostrophe alone after the plural S. The boys' backpacks. The dogs' tails. But not all plurals end in S. The children's backpacks. The women's college. The data's implications.

To form the possessive of these plurals, treat them like singulars, adding an apostrophe then an S. To put an S after one of these irregular plurals without an apostrophe is always an error. There is no "womens," "mens" or "childrens."

The complications don't end there. When you have a singular word that ends in S, like boss or James, publishers disagree on how to form the possessives. Some go even further and break these up into two groups: generic nouns like boss and proper nouns like James. That's why in a book, you might see "the boss's hat sat on James's head," but in a newspaper you could see "the boss's hat sat on James' head."

Some publishers also have special rules for nouns ending in S that precede words starting in S. So you could see "the boss's hat" on the same page you saw "the boss' seat" and both would be correct.

The apostrophe's other main job is to show that letters or numbers have been omitted, usually in contractions. "I've known him for years." "He's a great guy." "It'll be cold tomorrow." "Remember the '80s?" This all seems pretty straightforward until you remember that an apostrophe plus S is not just how we form singular possessives but also shorthand for the verb "is": It's raining.

On top of that, the plural of "it" does not have an apostrophe, just as his, hers and yours contain no apostrophe: "The dog wagged its tail."

As if all that weren't confusing enough, there's still one more thing to know about apostrophes: Even though it's normally a mistake to use one to form a plural, editing rules do allow apostrophes to form plurals when that's the only possible way to make clear what you're trying to say.

For example, if you were talking about student grades, an apostrophe would be crucial in a sentence like: "A's he earned in his classes look good on his permanent record." See how without the apostrophe it would appear we meant the word "as"? And imagine a sign on a storefront, written in all capital letters, announcing "DVD'S FOR SALE." In a lowercase sign, you would not use an apostrophe. But because the sign is in all caps, the apostrophe is the only way to make immediately clear you're not talking about something pronounced dee-vee-dee-ess.

When there's no way around it, you can use an apostrophe to form a plural. That's why the Chicago Manual of Style states specifically that there are apostrophes in "mind your p's and q's."

JUNE CASAGRANDE is the author of "The Best Punctuation Book, Period." She can be reached at JuneTCN@aol.com.