[QUOTE=Scarlett67]
I’d say “PO’s” falls into this category, and wouldn’t be uncomfortable styling it either way. (Though Chicago would lean toward omitting it, if an author preferred to keep it I would be OK with that.)
On preview: Colibri, is that from the 14th or 15th edition? (I can’t be arsed to dig out my copy of 14.) 15 has only the examples of Ph.D.'s and M.A.'s and x’s and y’s at para. 7.16; it doesn’t mention general confusion (although Amy Einsohn’s The Copyeditor’s Handbook [published before 15 came out] does).
[/QUOTE]
It’s the 13th Edition, actually, section 6.10, p. 160. I probably should update one of these decades.
However, I am not aware that the convention has changed significantly.
I also would in general not use an apostrophe to make a plural UNLESS confusion is likely. In the previous section, 6.9 in my edition, Chicago indicates this is the preferred method for letters used as words, acronyms, etc. However, to prevent confusion, using an apostrophe is an accepted practice by major style manuals. (Certainly Chicago style, and I assume the NY Times uses either AP style or their own house style.)
[QUOTE=Eleusis]
I say either “P.O.s” or “PO’s”
The apostrophe could account for the contraction of “Order”.
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Chicago style would make the first example “P.O.'s.” If you are using the apostrophe to indicate a contraction than the second example should be “P’O’s”; however, I have never seen that done and I doubt any style manual would accept it.
To be clear: although an apostrophe is typically used to indicate a contraction or a possessive, those are not its only uses. It can also be used, quite correctly, to indicate other spelling conventions or for clarity.