Plural Possessive—Bankers’ Boxes

I was asked this question today:

If John states in a report that he reviewed 3 “bankers boxes” of records, is the correct grammar “banker’s boxes,” “bankers’ boxes,” or “bankers boxes”? Should the “b” of bankers be capitalized? How about the “b” of boxes? Is it a proper name? What do you think?

Here is my response:

Since there are three bankers, bankers is plural, and to show possession for a plural word already ending in -s, you add the apostrophe after the -s:

three bankers’ boxes

Also, you should spell out three and any number less than 10—in most cases, anyway. There are some exceptions, though, like with ages.

Bankers’ boxes is not a proper noun, so no need to capitalize (unless Bankers’ starts a sentence, like it does in this instance).