If you type into Google's search engine the term "two chihuahua's," complete with apostrophe, you'll get plenty of hits that include the apostrophe. The search term "competing agenda's" also brings up a lot of apostrophe-laden hits, as does "news camera's."

This is one of the most common writing errors I see online: apostrophes used to form plurals.

This apostrophe error "continues to appear, to the amusement of educated people, in signs and notices, especially in shop windows," Fowler's Modern English Usage writes. And if Fowler were alive today, I'm sure he'd add Internet uses to that as well.

Yet somehow it still surprises me. In my little corner of the world, a book called "Eats Shoots and Leaves" got the final word on this matter more than nine years ago. The multimillion-selling book called to the attention of an entire hemisphere the "greengrocers' apostrophe" — the errant apostrophe used to form plurals like "carrot's" and "apple's" that famously show up in produce shop windows.

But my world isn't the whole world. I'm sure there are people who would be just as aghast that I don't know more about what's going on in space exploration, government or the music industry.

Still, anyone who wrongly uses an apostrophe to form a plural runs the risk she'll evoke eye rolls or unkind thoughts from readers. With just a quick overview of the apostrophe rules, anyone can learn to avoid this most iconic punctuation error.

Apostrophes are used to form possessives, as in "the dog's tail." They also form contractions, as in "Here's looking at you, kid."

But they have a third use that, I suspect, is responsible for much of the confusion. According to most major style guides, apostrophes can be used to form plurals if failure to do so would create confusion. For example, if you're telling someone to mind his p's and q's, the Chicago Manual of Style says to use apostrophes. It makes sense. "Mind your ps and qs" just isn't as clear.

Of course, when you're using capital letters to talk about letters, this often isn't necessary: "Mind your Ps and Qs." But what if you were talking about the grades of a child who usually gets Bs but suddenly starts doing better?

Would you worry that As, unlike Bs and Cs, actually creates a word, "as"? The Los Angeles Times does. Which is why their official style guide calls for the punctuation shown in this sentence: "Billy gets mostly Bs and Cs and a few A's." That's right, The Times has a different rule for A than for B.

Other publications and style guides would say to put the apostrophe in B's and C's, too, for consistency's sake. While still others say to go ahead and let your "As" look like "as." There's no single right way to write these plurals, so publications make their own decisions.

Plurals of numbers can also be confusing. Most publications will use no apostrophes in decades, such as the 1980s or 1820s. But open the New York Times and you'll see that practice turned on its head: That paper writes them with apostrophes: 1980's and 1820's.

Those are just some of the reasons why apostrophe use can seem like a minefield. But you can navigate the rules with this simple guideline: Never use an apostrophe to form a plural unless the alternative would mean confusion, as in an all-capital-letter sign announcing DVD'S for sale.

And don't let words or even proper names that end with vowels throw you. Mr. and Mrs. Miceli are the Micelis. One video plus another video make two videos. No apostrophe needed.

JUNE CASAGRANDE is the author of "It Was the Best of Sentences, It Was the Worst of Sentences." She can be reached at JuneTCN@aol.com.