Not too long ago, my friend Josh spotted an advertisement that went to print with a misused apostrophe, so he cut it out and mailed it to me. The headline read, "Train like the Pro's."
Josh found what grammarian Lynne Truss calls the "greengrocer's apostrophe," so named because of those handwritten signs in markets that announce Carrot's or Apple's. No apostrophe is needed to pluralize these words.
In fact, the apostrophe is hardly ever used for plurals. The apostrophe indicates possession (e.g. Josh's letter) or missing characters, as in contractions (don't, won't) or in numeric references (hits of the '90s). The apostrophe has a few other minor roles, but these two tasks are the big ones.
There are a lot of times when using the apostrophe to make a plural seems needed, but isn't. Referencing decades doesn't require an apostrophe; it's simply 1990s. Same thing goes for initialisms: CDs, DVDs and PCs, for example. Putting family names in the plural doesn't require apostrophes, either; Morrissey and Marr had it right when they called their band The Smiths. For days of the week, just add s: rainy days and Mondays. And words ending in vowels don't use apostrophes to become plural. The word pro simply becomes pros. Words like tomato take on es to become tomatoes.
The rare times when apostrophes are used for plurals happen in specific phrases involving certain well-worn letters or words, as in "Customer service do's and don'ts." That's really about it. Meanwhile, gross words like "bacterium" and "fungus" have specific rules about plurals, none of which use the apostrophe.
The best rule of thumb is to avoid apostrophes when pluralizing words and just add s or es. It saves time. It saves keystrokes. It may save Josh the cost of a postage stamp.
Sources: Eats, Shoots & Leaves by Lynne Truss; The
Elements of Style by William Strunk, Jr. and E.B. White; The Bloomsbury Grammar Guide by Gordon Jarvie; The Gregg Reference Manual by William A. Sabin.
Songs from this Episode: "Act of the Apostle" by Belle and Sebastian; "Spin the Bottle" by the Juliana Hatfield Three; "Panic" by the Smiths