A greengrocer’s apostrophe occurs whenever a writer attempts to pluralize a word by using an apostrophe plus “s” instead of the proper plural ending. A handmade sign in a local grocery store might advertise “Apple’s Two Dollars a Pound” or “Orange’s $3.99 a Bag”, for example. The term “greengrocers’ apostrophe” was actually inspired by such prominent grammatical errors in grocery store signs.
The ill-conceived practice of using a greengrocer’s apostrophe is not limited to grocery stores, however. Many local or small businesses have been known to create similar signs advertising “Stereo’s and Television’s On Sale” or “Compare Our Rate’s With Other Company’s!” Instead of properly pluralizing “stereos,” “televisions,” “rates” or “companies,” the signmaker simply used a greengrocers’ apostrophe.
An apostrophe S (or S apostrophe) generally denotes possession, not number. It can also be used in a contraction, such as “it’s” for “it is.” There are few exceptions to the rule against using a greengrocers’ apostrophe to pluralize, however. In some cases, an apostrophe might be used to indicate a plural of a letter, such as “A’s” and “U’s” although some grammarians argue against this use as well; without the apostrophe, however, those letters could be confused for the words “As” and “Us.”
I do have more on apostrophes on my blog – if you use the search function you will find them.