Originally Posted by D

K, pulling the memory from my writing instruction...

Lowercase letters and acronyms or abbreviations with punctuation at the end (example: "m," "k," "P.H.D.," "C.I.A.," and "in.") use an apostrophe before the "s" when making them plural.

But you really shouldn't use periods in acronyms and abbreviations, anyways, unless it's to avoid confusion (such as in "in." for "inches", as "in" is actually a word with another meaning).

Technically, it wouldn't be proper in the case of capital letters and acronyms or abbreviations without punctuation (example: "M," "K," "PHD," "CIA," and "in"), but it's often used.

So, technically, "M's" - though used often - would be incorrect. "Ms" would be correct. "M&Ms" is actually correct while "M&M's" is, technically, incorrect, I believe

The plural, "m's," would be correct, and "ms" would be incorrect, no matter how you slice it. Same for "m&m's" (correct) and "m&ms" (incorrect).



Still, I, personally, find myself applying apostrophes to the end of acronyms and abbreviations all the time, even though it's not, technically, correct to do so... it's kinda become accepted (though incorrect) practice for all letters, acronyms, and abbreviations when, in fact, it's only correct for lower-case letters and abbreviations or acronyms ending in a period.