One unusual use of the apostrophe is in plural acronyms, like “ICBM’s” “NGO’s” and “CD’s”. Since this pattern violates the rule that apostrophes are not used before an S indicating a plural, many people object to it. It is also perfectly legitimate to write “CDs,” etc. See also “50’s.” But the use of apostrophes with initialisms like “learn your ABC’s and “mind your P’s and Q’s” is now so universal as to be acceptable in almost any context.
Note that “acronym” was used originally only to label pronounceable abbreviations like “NATO,” but is now generally applied to all sorts of initialisms. Be aware that some people consider this extended definition of “acronym” to be an error.
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