The greengrocers' 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 signage.
The ill-conceived practice of using a greengrocers' 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!"
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."
When the term greengrocers' apostrophe was first popularized, most likely in the Liverpool, England, area during the 1950s, there were a number of foreign-born store owners who overused apostrophes in an effort to over-correct their English grammatical errors in general. A pluralized English word and a possessive can sound exactly alike, so those learning the language may use plurals as possessives and possessives as plurals.
The rules governing the proper use of apostrophes can be quite confusing until the basic differences between plurals and possessives are fully understood. It can be difficult to explain why a sentence such as "My sister's friends did not like the movie." is not the same as "My sisters' friends did not like the movie, either." Possession and number are very closely connected when it comes to the proper use of apostrophes, so mistakes such as the greengrocers' apostrophe are understandable during the learning process. As a student's understanding of English grammar rules improves, the use of the greengrocers' apostrophe should start to become less frequent.