Q: I know a letter or number gets an apostrophe when made plural: x’s, 9′s, and so on. But what happens when letters make up an abbreviation: CEO, RN, MD, and so on? Does the abbreviation get an apostrophe when made plural?
A: There’s no single “rule” about this, since conventions vary widely from publisher to publisher, usage guide to usage guide.
On our blog, we generally follow The Chicago Manual of Style (16th ed.). Here’s what it recommends:
“Capital letters used as words, numerals used as nouns, and abbreviations usually form the plural by adding s. To aid comprehension, lowercase letters form the plural with an apostrophe and an s.”
The Chicago Manual gives these examples: “the three Rs … x’s and y’s … the 1990s … IRAs … URLs … BSs, MAs, PhDs.”
Unlike you, the Chicago Manual would not use an apostrophe with 9s. And as you can see from the examples above, it would not use apostrophes in CEOs, RNs, MDs, and so on.
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