Q. I was horrified to see that you endorsed using an apostrophe before the s to form plurals! “To aid comprehension, lowercase letters form the plural with an apostrophe and an s (compare ‘two as in llama’ with ‘two a’s in llama’)” (CMOS 7.15). I protest. An apostrophe conveys possession, or a contraction. It should never be used in this context. Please advise where this misbegotten rule came from.

A. The nice thing about using an apostrophe to help form a plural is that it does it so well; you’d never know that it was born under questionable circumstances, or that it doesn’t have a right to play that role. You’ll find it in Shakespeare: “By my life this is my Ladies hand: these bee her very C’s, her V’s, and her T’s, and thus makes shee her great P’s. It is in contempt of question her hand” (Twelfth Night, act 2, scene 5 [1st folio, 1623]; and note the absence of an apostrophe, and the plural ending, in the possessive “Ladies”). In its first eleven editions, CMOS advised writing “the three R’s,” after which it became “the three Rs.” But the intent of the rule has remained the same: use an apostrophe wherever it is needed to prevent a misreading. And as anyone who got A’s in chemistry (or knows their Agatha Christie) might tell you, sometimes an apostrophe can spell the difference between a letter grade and a poison.