The Apostrophe with Numbers, Letters, and Abbreviations
Rule 1: The plurals for capital letters and numbers above nine do not require apostrophes, but some use them anyway.
She learned her ABCs. (some writers prefer ABC’s)
the 1990s (some writers prefer 1990′s)
Rule 2: For clarity, most writers use apostrophes with single capital letters and single-digit numbers.
Please dot your I’s.
She learned her times tables for 6′s and 7′s.
Posted on Monday, November 9th, 2009, at 10:28 am