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Where does the practice of using -a and -i for plural forms of -um and -us, respectively, come from?

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RegDwigнt
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Arlen Beiler
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3 Answers3

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These words have these plurals because they are loan words from Latin. Words that come from Latin that end in -um usually have plurals in -a, while those that end in -us have plurals in -i. This way of forming plurals is normal in Latin, and learned English preserves the native Latin plurals.

waiwai933
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JSBձոգչ
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  • Good observation about the ending, I missed that. – Arlen Beiler Aug 10 '10 at 11:23
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    Key word: "usually." Some words that end in -us that are from Latin might have plurals in -i or -us, depending on the declension. – John Calsbeek Feb 04 '11 at 23:15
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    Circus and campus are good examples of Latin words that have regular English plural forms. – Kosmonaut Apr 13 '11 at 22:14
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    And a few words end in -us without coming from Latin (octopus and platypus, for instance). – Henry Apr 13 '11 at 22:46
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    octopus and platypus are from greek. Plurals are octopodes and platypodes. – Michael Brown Dec 02 '11 at 15:17
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    I am reminded of the classics professor who was mugged by two hoodla ... – Jay Jan 02 '12 at 17:34
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    Beware the 3ʳᵈ declension: corpus > corpora, genus > genera, opus > opera, tempus > tempora – tchrist Mar 18 '12 at 04:36
  • Likewise status is of the fourth declension as my boss forcefully reminded me when I once tried to report my stati :-) – Pete Wilson Apr 13 '11 at 22:22
  • @MikeBrown But octopi and platypi are acceptable plural forms. Basically there's not much point to keeping track of whether a word is Greek or Latin if it ends in something like us; you might as well pluralize using i since people will make that mistake and then it becomes common usage – Chan-Ho Suh Aug 18 '12 at 01:03
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    @Chan-HoSuh That doesn’t make sense. You might as well pluralize corpus > corpi, genus* > geni, opus* > *opi then — which is nonsense. What’s O tempi O morrises then? A lament concerning morris dancers who can’t keep good time? :) – tchrist Aug 18 '12 at 03:34
  • Good examples! I should have put a smiley after my comment; it was meant somewhat tongue-in-cheek. – Chan-Ho Suh Aug 18 '12 at 04:12
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    @Chan-HoSuh, if you’re going to argue that keeping track of etymologies and original plurals is persnickety and somewhat pointless (which is a valid argument), at least argue for creating regular plurals for them, rather than just making up strange ones. ‘Octopuses’ and ‘platypuses’ are both infinitely preferable to ‘octopi’ and ‘platypi’, the mere typing of which just caused my fingers to go into apoplectic spasms of etymological epilepsy. – Janus Bahs Jacquet Sep 08 '13 at 11:59
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    @JanusBahsJacquet "...just making up strange ones"? I didn't make it up though, as I think you are well aware. – Chan-Ho Suh Sep 09 '13 at 00:15
  • @Chan-HoSuh, no, I meant rather than arguing for the practice of making up these horrible hybrid plurals, not making them up yourself. :-) – Janus Bahs Jacquet Sep 09 '13 at 07:45
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    @JanusBahsJacquet Well, I wasn't arguing for that per se. I was arguing for ambivalence. – Chan-Ho Suh Sep 11 '13 at 03:39
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It comes from people who still remember that a word is a loan word and the lending language was inflected. Often people attempting to inflect the way Latin does do a poor job of it, so outside of the most common Latinisms, it would be better style to use ordinary plurals.

MatthewMartin
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1

These words are loan words from Latin. The plurals associated with words ending in -um or -us are not dictated by practice, but by precise, Latin, rules.

In Latin - which is an inflected language - there are 5 declensions. Nouns are distributed among declensions and follow declension-specific rules.

So, a noun belonging to the second declension and ending in -us (such as lupus), will have lupi as plural, while one belonging to the same declension and ending in -um will have an -a plural (bellum -> bella).

Note that in Latin nouns have a gender, so lupus is male, while bellum is neuter.

A noun belonging to the fourth declension such as spiritus (male) will have spiritus as plural.

Janus Bahs Jacquet
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Roberto Aloi
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    I wouldn't say the plurals are dictated by precise rules. On the contrary, I would say they are dictated by inconsistently applied rules that were, at some stage, informed by Latin pluralization. We have Latin words that have regular English plurals (campus pluralizes as campuses), Latinesque plurals for words people think are Latin that aren't, like octopus-octopi (octopus is Greek), words that were originally plural in Latin that are now mass nouns (data), and words where the Latin plural form is often used for singular and plural (alumni). – Kosmonaut Apr 13 '11 at 22:19
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    @Kosmonaut -- just my opinion, but anyone who uses "alumni" as a singular should be shot into space... – Michael Lorton Apr 13 '11 at 23:38
  • The precise rules statement referred to the Latin plurals, not to the English ones :) – Roberto Aloi Apr 14 '11 at 14:06
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    @Malvolio If you want to extend that to Italian, anyone who uses panini as a singular word should be shot. – Roberto Aloi Apr 14 '11 at 14:07
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    people say "a panini"? OK, I've never heard that but yes, anyone saying it will be condemned to live on Wonder Bread and yellow mustard for the rest of his life. How do people feel about 'an agenda'? – Michael Lorton Apr 14 '11 at 14:31
  • @Malvolio Alumni is a perfectly fine singular form — in the genitive, that is. It means alumnus’s. – tchrist Aug 18 '12 at 03:36
  • @tchrist: if we were speaking Latin, which we aren't, so it's not a perfectly fine form in the singular. – siride Sep 07 '13 at 23:47
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    @RobertoAloi, even in Latin, the rules are far from precise—they are, in fact, full of inconsistencies and irregularities. And as Kosmonaut said, there are plenty of words in English that are taken directly from Latin, but are not declined as in Latin. You even mentioned one of them in your answer: ‘lupus’, whose plural, though rare, is ‘lupuses’. – Janus Bahs Jacquet Sep 08 '13 at 12:08
  • I had been wondering why strata is the plural form of stratum, as well as where the word originated, and I found the answer(s) here. Its etymology in Dictionary.com would be from Latin strāta. See also these pages: https://english.stackexchange.com/questions/441504/why-is-genera-the-plural-form-of-genus; https://english.stackexchange.com/questions/536822/why-is-stigmata-a-plural-of-stigma – Adamant Dec 28 '20 at 19:24