We use an apostrophe to create plural forms in two limited situations: for pluralized letters of the alphabet and when we are trying to create the plural form of a word that refers to the word itself. Here we also should italicize this "word as word," but not the 's ending that belongs to it. Do not use the apostrophe+s to create the plural of acronyms (pronounceable abbreviations such as laser and IRA and URL) and other abbreviations.

A possible exception to this last rule is an acronym that ends in "S":

• Jeffrey got four A's on his last report card.
  
  
• Towanda learned very quickly to mind her p's and q's.
  
  
• You have fifteen and's in that last paragraph.

Notice that we do not use an apostrophe -s to create the plural of a word-in-itself. For instance, we would refer to the "ins and outs" of a mystery, the "yeses and nos" of a vote (NYPL Writer's Guide to Style and Usage), and we assume that Theodore Bernstein knew what he was talking about in his book Dos, Don'ts & Maybes of English Usage. We would also write "The shortstop made two spectacular outs in that inning." But when we refer to a word-as-a-word, we first italicize it — I pointed out the use of the word out in that sentence. — and if necessary, we pluralize it by adding the unitalicized apostrophe -s — "In his essay on prepositions, Jose used an astonishing three dozen out's." This practice is not universally followed, and in newspapers, you would find our example sentence written without italics or apostrophe: "You have fifteen ands in that last paragraph."

Some abbreviations have embedded plural forms, and there are often inconsistencies in creating the plurals of these words. The speed of an internal combustion engine is measured in "revolutions per minute" or rpm (lower case) and the efficiency of an automobile is reported in "miles per gallon" or mpg (no "-s" endings). On the other hand, baseball players love to accumulate "runs batted in," a statistic that is usually reported as RBIs (although it would not be terribly unusual to hear that someone got 100 RBI last year — and some baseball commentators will talk about "ribbies," too). Also, the U.S. military provides "meals ready to eat" and those rations are usually described as MREs (not MRE). When an abbreviation can be used to refer to a singular thing — a run batted in, a meal ready-to-eat, a prisoner of war — it's surely a good idea to form the plural by adding "s" to the abbreviation: RBIs, MREs, POWs. (Notice that no apostrophe is involved in the formation of these plurals. Whether abbreviations like these are formed with upper- or lower-case letters is a matter of great mystery; only your dictionary editor knows for sure.)

Notice, furthermore, that we do not use an apostrophe to create plurals in the following:

• The 1890s in Europe are widely regarded as years of social decadence.
  
  
• I have prepared 1099s for the entire staff.
  
  
• Rosa and her brother have identical IQs, and they both have PhDs from Harvard.
  
  
• She has over 400 URLs in her bookmark file.