orthography - Plurals of acronyms, letters, numbers — use an apostrophe or not?

When I was in high school back in the 1970s, I was taught that to make a plural of an acronym, a letter, or a number, one should add an apostrophe and "s". Like I would have written this sentence, "... back in the 1970's ..." I would write "one CD, two CD's". Etc. I followed this rule faithfully for years until a co-worker told me it was wrong. Now I can't find any source that agrees with what I was taught.

Is this a rule that has changed over time? Was the convention in the 70s that one should use an apostrophe but this has changed and now we do not? Or were my high school English teachers just confused?

Answer

I seem to remember the old askoxford.com site said either was acceptable: CDs and CD's.

But now the replacement Oxford Dictionaries Online firmly suggests to avoid the apostrophe except in a few special cases:

Apostrophes and plural forms

The general rule is that you should not use an apostrophe to form the plurals
of nouns, abbreviations, or dates made

up of numbers: just add -s (or -es, if the
noun in question forms its plural with -
es). For example:

...
MP
MPs (e.g. Local MPs are divided on this issue.)

1990
1990s
(e.g. The situation was different in the

1990s.)

It's very important to remember this grammatical rule.

There are one or two cases in which it is
acceptable to use an apostrophe to
form a plural, purely for the sake of
clarity:

• you can use an apostrophe to show the plurals of single letters:
  
  
  
  
  • I've dotted the i's and crossed the t's.
  
  
  • Find all the p's in appear.
  
  


• you can use an apostrophe to show the plurals of single numbers:
  
  
  
  
  • Find all the number 7’s.
  
  
  

These are the only cases in which it is
generally considered acceptable to use
an apostrophe to form plurals:
remember that an apostrophe should never be used to form the plural of ordinary nouns, names, abbreviations,
or numerical dates.