What are the rules for using apostrophes after an acronym that ends in the letter "S"?

For instance, if I have a pretend product called Fast Ice Cream System, or FICS, which delivers ice cream very quickly to wherever you are, I could:

  • Have more than one FICS (would this be written as FICSs, FICS', FICS'S or FICSS)
  • Have a component of the FICS (this would be "the FCIS's flux capacitor)

Is there an appropriate English style guide? I can't imagine this is new territory.

  • @EdwinAshworth my question is specifically about abbreviations that end in S, it’s not clear to me if the rules differ Oct 21, 2021 at 18:37
  • From Pain in the English: << This rule is exactly the same when the acronym in question ends in an S. So you have one ACS, and a thing belonging to it is the ACS's thing. Or you have additional ACSs, and something belonging to the ACSs is the ACSs' thing. ... speedwell2 >> However, the majority view is that abbreviations not pronounced as if single words are initialisms, not acronyms. Thus CIA (and doubtless FICS) is an initialism, NASA (like radar) an acronym Oct 22, 2021 at 11:30


Browse other questions tagged or ask your own question.