What is the proper way to pluralize acronyms ending in the letter 'S'?
The particular acronym under study in this case is T.A.R.D.I.S.
The various forms I believe are correct are:
- T.A.R.D.I.S.es (is it even correct to pluralize this)
Whoniverse.net and Wikipedia both use TARDISes. This follows general pluralization rules and seems to generalize to pluralizing other acronyms, e.g. radars and lasers, as well as to initialisms such as USBs, GPSes.
It seems that keeping the capital letters for acronyms, as opposed to initialisms, is somewhat uncommon, but we have WASPs, for instance.
Not everyone does, but I'm using the distinction that acronyms are pronounced as words and initialisms are instead pronounced as individual letters, as supported by Ben Zimmer in the New York Times Magazine.
The Guardian style guide says:
Hence, it wouldn't be T.A.R.D.I.S. or TARDIS, but Tardis, and therefore, pluralised, Tardises.
However, the Guardian style guide is only one guide. Other publications may have different rules, and if nobody is making the rules for you, you can choose your own.
The general approach is to derive the plural according to how an abbreviation, acronym etc., is pronounced when read aloud.
If T.A.R.D.I.S. is the usual format in writing so that the alphabets are pronounced separately, then it is not advisable to pluralize it at all. You can circumvent this by such alternatives as instances of T.A.R.D.I.S. or such other constructions.
On the other hand, if TARDIS is the usual way it is written and pronounced like a word, then TARDISes would be acceptable and comprehensible to the average reader.