Q: I am an academic scientist, and I often need to add an “s” to pluralize an abbreviation. I think one should be able to put an apostrophe before the “s” so as not to add to the confusion inherent in the abbreviation. For example, one refers to runs of adenine and thymine bases in DNA as ATs, but AT’s seems clearer. Even more confusing is a mixed case example like RNAi (interference RNA): I would like to pluralize it as RNAi’s. Thank you for any input.
A: Many people are violently against the use of apostrophes in plurals – any plurals, even abbreviations, numbers, and individual letters. But here, I think, we have to bow to readability rather than blindly follow rules that are mere stylistic conventions anyway.
This is what I say in the relevant paragraphs from the new third edition of my grammar and usage book Woe Is I:
“Over the years, authorities have disagreed on how we should form the plurals of abbreviations (GI, rpm, RBI), letters (x, y, z), and numbers (9, 10). Should we add s, or ’s? Where one style maven saw UFO’s, another saw UFOs. One was nostalgic for the 1950’s, the other for the 1950s.
“The problem with adding ’s is that we get plurals and possessives confused. Is UFO’s, for example, a plural (I see two UFO’s) or a possessive (That UFO’s lights are violet)?
“Here’s what I recommend, and what most publishers do these days. To form the plurals of abbreviations and numbers, add s alone, but to form the plural of a single letter, add ’s. CPAs, those folks who can add columns of 9s in their heads, have been advising MDs since the 1980s to dot their i’s, cross their t’s, and never accept IOUs. Things could be worse: there could be two IRSs.
“Why use the apostrophe with a single letter? Because without it, the plural is often impossible to read. Like this: The choreographer’s name is full of as, is, and us. (Translation: His name is full of a’s, i’s, and u’s.)”
Although the two examples you cite don’t involve single letters, I agree with you that readability should be a consideration. With that in mind, I think the lowercase “i” in RNAi should be followed by an apostrophe and “s” when pluralized. Ditto for other mixed-case abbreviations, even if the lower-case letters aren’t at the end.
But I’m of two minds about pluralizing AT and other all-cap scientific abbreviations. In general, I think “s” alone would suffice for the plural (ATs). But one might want to use apostrophes for consistency when citing both all-cap and mixed-case abbreviations in the same paper.
Unlike the rules for making nouns possessive, the ones for making unusual nouns plural are not written in stone and not universally agreed upon. Details of punctuation may differ from publisher to publisher and from country to country (American and British practices differ, for example).
In the case of a scientist like you, who may use scores of abbreviations at a time, there’s a lot to be said for consistency. If you want to use apostrophes to pluralize scientific abbreviations, go for it.