great question... Here is an explanation I found on the website grammarbook.com. Let me know if you have any questions about what they say: Many writers and editors prefer an apostrophe after single capitalized letters. Example: I made straight A's. With groups of two or more capital letters, apostrophes seem less necessary. Examples: There are two new MPs on the base. He learned his ABCs. She consulted with three M.D.s. OR She consulted with three M.D.'s. Some write M.D.'s to give the s separation from the second period. Single-digit numbers are usually spelled out, but when they aren't, you are just as likely to see 2s and 3s as 2's and 3's. With double digits and above, many (but not everyone) regard the apostrophe as superfluous: I scored in the high 90s. There are different schools of thought about years and decades. The following examples are all in widespread use: Examples: the 1990s the 1990's the '90s the 90's Awkward: the '90's

The most common approach is to avoid the apostrophe except for lowercase letters (as you said above) and for numbers like "6's." CDs, MCs, and just about everything else takes an s, and no apostrophe.