How does one determine the correct plurals of "made up" words that have no given plural form given by its original canon source or creator? Is it possible to determine what the "most similar" existing word would be, and mimic its own rules for making plurals?

For example, one wished to determine the plural form of koopa, a species from the Super Mario Bros universe, be it either koopas or koopa, would there be a way to tell by looking at similar real English words, such as "sheep" or "mouse" (which are both names of species of creatures, like the fictional word, and have special plurals)? Or would the only way to know be to ask Shigeru Miyamoto or another source of Nintendo of America? (Please note that this is only an example, and does not itself need answering)

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    The author determines, according to whatever rules and purposes they may have in mind. It's not like it's covered by statute law, you know. People can do anything when they're making up a language.
    – jlawler
    Commented Apr 19, 2013 at 0:07
  • I think this question might be more suitable on one of the other SE sites. I'm going to ask about migrating it. (I think you've asked the question well, it's just not really about English language. Maybe Writers or Linguistics can help.)
    – Kit Z. Fox
    Commented Apr 19, 2013 at 0:12
  • @JohnLawler I am not talking about entire languages, but rather individual, single made-up words when used in English, such as if someone invented the name of a fictional race
    – Southpaw Hare
    Commented Apr 19, 2013 at 0:23
  • Yes, there are concrete rules about these things. Best follow them. I prefer the i-mutation plurals of Sindarin myself.
    – tchrist
    Commented Apr 19, 2013 at 0:50
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    @Anixx The only part specific to English is the example. Even if it wasn't the OP's original intent, it isn't hard to read the question as written in a language-generic fashion. Furthermore, looking at the types of answers this question is provoking it is undoubtedly a good fit here. Finally, nothing in the FAQ says that language-specific is forbidden, just that we need to be extra careful about the type of questions we accept. If the question was "how should we create plurals" I'd agree that it's off topic but the question seems to be asking how we naturally form the plurals.
    – acattle
    Commented Apr 20, 2013 at 1:33

2 Answers 2


Research in linguistics has shown that when English-speaking children are asked to pluralise made-up words (which fit with the phonotactic constraints of English) they use the standard productive plurals, suffixing:

  • /əz/ on words with final /s, ʃ, ʧ, ʤ, z/;
  • /s/ on words with an final /p, t, k, f, Ө/;
  • /z/ on words with all other finals, including vowels

It seems that under normal circumstances English speakers would apply this rule to novel words, but whoever makes up the word can make up the plural as well. An example of this is the semi-jocular plurals of 'Unix' as 'Unices' and 'Unixen', instead of the predicted 'Unixes' /ˈjunɪksəz/.

For the example you give, 'koopa', the expected plural form would be 'koopas', pronounced something like /kupəz/.

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    That is, to say, English speakers use English plurals unless otherwise directed. Not surprising, but definitely the benchmark to measure from.
    – jlawler
    Commented Apr 19, 2013 at 3:46

Made up words which are used in English will follow pluralization rules that hold for other English words. See the Wikipedia entry on Wug Test. The most reasonable plural of koopa is koopas, since most English nouns form their plurals by adding (orthographic) -s. If you want to take an affected air, you can call them koopae, by analogy with Latinate English words ending in -a which have a learned plural form with -ae (e.g., alumna, alumnae, which you might find in some brochures for women's colleges).

You couldn't get koopa to have another type of irregular plural (and expect people to guess you are talking about more than one kooopa), unless the singular form has the right sound pattern that will let people pick up on your game.

  • The Wug Test is very relevant and useful! Thank you.
    – Southpaw Hare
    Commented Apr 19, 2013 at 1:02
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    No, KOO is not a Latin letter combo, so -AE fails. You have to follow established patterns, or devise your own in a hyperconsistent manner, or else it doesn’t work.
    – tchrist
    Commented Apr 19, 2013 at 1:03
  • Koopa is a reasonable form for a faux Japanese loanword, though.
    – Bradd Szonye
    Commented Apr 19, 2013 at 4:57
  • @tchrist cup.ae N 1 1 GEN S F cup.ae N 1 1 LOC S F cup.ae N 1 1 DAT S F cup.ae N 1 1 NOM P F cup.ae N 1 1 VOC P F cupa, cupae N F [XAXFO] veryrare bar of an oil press (axle for millstones); axle; crooked handle (L+S); barrel, cask, vat, tun; (esp. for wine); niche in a columbarium (for ashes); dancing-girl; female tavern-keeper and castanet-dancer (L+S); female vintner;
    – user483
    Commented Apr 19, 2013 at 22:33
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    But since many fewer people know anything about Latin than know that some English words in -a have a plural in -ae, the fact that koo doesn't exist in Latin is pretty irrelevant, really.
    – Colin Fine
    Commented Apr 21, 2013 at 0:30

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