Plurals of Abbreviations, Letters, and Numbers?
“No two authorities seem to agree on how we should form the plurals of abbreviations (GI, rpm, RBI), letters (x, y, z), and numbers (9, 10). Should we had s or ‘s? Where one style maven sees UFO’s, another sees UFOs. One is nostalgic for the 1950’s, the other for the 1950s. This is more a matter of taste and readability than of grammar, and frankly, we have better things to worry about. For the sake of consistency and common sense, here’s what I recommend. To form the plurals of all numbers, letters, and abbreviation (with or without periods and capitals) simply add ‘s” (O’Conner, Woe Is I 30).
