The Apostrophe with Numbers, Letters, and Abbreviations

Rule: The plurals for capital letters and numbers used as nouns are not formed with apostrophes.

Example: She consulted with three M.D.s.
BUT
Example: She went to three M.D.s’ offices.
The apostrophe is needed here to show plural possessive.
Example: She learned her ABCs.
Example: the 1990s
NOT
the 1990’s
Example: the ’90s or the mid-’70s
NOT
the ’90’s or the mid-’70’s
Example: She learned her times tables for 6s and 7s.
Exception: Use apostrophes with capital letters and numbers when the meaning would be unclear otherwise.
Example: Please dot your I’s.
You don’t mean Is.
Example: Ted couldn’t distinguish between her 6’s and 0’s. You don’t mean Os.