In computer science we discuss an abstract machine called a "deterministic finite automaton". The standard initialism for this term is "DFA". This makes sense in the singular usage of the initialism.

However, the pluralization of the word "automaton" is "automata". One speaks of "an automaton" or "many automata". Consequently, it seems intuitive to form an initialism of the phrase "deterministic finite automata" as "DFA".

This does not seem correct, as "DFA" could be either plural or singular. On the other hand, "DFAs" also does not seem correct because it would seem to expand to "deterministic finite automatons", a phrase which no self-respecting computer scientist would ever utter.

This appears to be a duplicate of the question here What is the plural of the abbreviation of "multiplicity automaton", "MA" or "MAs"?

But that question was never clearly resolved. What to do?

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    What's intuitive isn't always what's used. Consider that in baseball we pluralize the already-pluralized RBI (runs batted in) by saying RBIs. Some broadcasters use RBI for both singular and plural and the verb gives away the plurality.
    – tylerharms
    Commented Mar 10, 2017 at 20:13
  • I understand that. But what I would like is to be able to find some rule in some style guide that covers this case. For instance, I would like to see a rule that says "all initialisms are pluralized by adding the letter 's', regardless of the pluralizations of the constituent words". Is there any such rule?
    – user10108
    Commented Mar 10, 2017 at 20:31
  • In a related thread at WordReference.com: << "Could you please tell me the plural for M.A. as in the degree."_fofoca, Nov 15, 2007 // "... If you don't use periods, just add "s": MAs It is increasingly common and considered improved style to drop the periods from abbreviations." _ Matching Mole Senior Member England >> Note that MsA might be expected, but that the initialism/acronym is treated as a quasi-word hereabouts. Compare MPs. (And be aware that 'acronym' is used with conflicting senses.) Commented Mar 10, 2017 at 20:40
  • I can't imagine there is a rule, but you might find a style guide to lean on. For example, here's from Johns Hopkins style manual. Their guide doesn't address your query but it does deal with initialisms as if they were their own entity.
    – tylerharms
    Commented Mar 10, 2017 at 20:42
  • 1
    @EdwinAshworth, it is incredibly uncommon in computer science to use "automatons" over "automata". If you submit a paper for review that uses the word "automatons" you will definitely be scolded by the reviewer.
    – user10108
    Commented Mar 10, 2017 at 21:13

2 Answers 2


In reading a few sources--a blog, the Chicago Manual of Style FAQ and the Johns Hopkins Style Guide--on how they handle initialisms, the understanding I've taken is essentially that you should treat the initialism as its own entity, apart from the words it represents. As the Chicago Manual says:

If you can stop thinking of the spelled-out meaning of the acronym and just treat the acronym itself as a word with its own meaning, you should be able to add that little s without fretting.

They are responding to a question about an initialism where the final word is already plural, which is not what the OP is asking, but the advice extends: treat the initialism apart from the word. And, in this case, if no self-respecting computer scientist would ever utter deterministic finite automatons then expect that they would probably correctly infer that DFA followed by plural verb agreement would recall the appropriate terminology.

  • Thanks for the sources. This reasoning is intuitive and I agree with the recommendation.
    – user10108
    Commented Mar 10, 2017 at 21:52
  • 'Treat it as a word' is more relevant for acronyms than for initialisms. However, when it comes to forming plurals, this crude rule works. Commented May 9, 2020 at 18:42

Think of it this way...

While talking shop with other computer scientists, you refer to this concept: DFA.

A DFA is a "thing" you all know. Refer to the plural of DFAutomaton by the way you pluralize the abstract "thing," "things."

DFA == Deterministic Finite Automaton

DFAs == Deterministic Finite Automata

DFAs ≠ Deterministic Finite Automatons, because you are a self-respecting computer scientist.

In this way, you easily communicate which one it is to which you are referring, and you can live with yourself.

  • 1
    To quote @Matt Gutting: "Hi! What we're really looking for (on this or any other Stack Exchange site) is a supported answer; one that you can support with authoritative references (in this case an encyclopedia, dictionary, or some other such document). Edit your question and put in your support; then we'll be able to vote up your answer!" Your answer sounds like it uses a sensible approach – but this is English. Answers given on ELU which lack supporting references come across merely as opinion. Commented Mar 10, 2017 at 20:51
  • I like the answer, and this is how I have intuitively been using it, but, as mentioned by @EdwinAshworth, I would like supporting documentation.
    – user10108
    Commented Mar 10, 2017 at 21:15
  • Where does one find authoritative supporting documentation on pluralization of an English acronym containing a Latin singular or plural form? No matter; I have another opinion: use DFA for both singular and plural, knowing what it means by context. If context isn't enough in certain cases, use the full term. Disclosure: I'm both a computer scientist and native speaker of English.
    – R Morrison
    Commented Mar 11, 2017 at 21:57
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    For support I offer examples of other initialisms which pluralise using final s (and in no other way) even though, in the full form, the head is not the last word: Members of Parliament are MPs; curricula vitae are CVs; sites of special scientific interest are SSSIs; Masters of Science and Master of Science degrees are MScs.
    – Rosie F
    Commented Dec 6, 2018 at 16:55

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