I'm not familiar with irregular Latin pluralization, so this may be a simple question with a simple answer. Other Latin words ending in "us" don't pluralize to "era"

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    See en.m.wikipedia.org/wiki/Latin_declension. Genus is a third-declension neuter noun. Don't think of the -era as being the plural of -us; rather, think of gener- as being its own stem used in certain forms of genus. (This is not considered irregular in Latin, even if it seems irregular by English standards.)
    – ruakh
    Commented Apr 13, 2018 at 12:50
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    I'm voting to close this question as off-topic because it's technically about Latin, not about English, even though English has borrowed these words. (There is a Latin SE).
    – cobaltduck
    Commented Apr 13, 2018 at 14:17
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    It's just as much a question about English as is the question of why the plural of "child" is "children". The fact that the answer re "genus" involves Latin, or even that the OP anticipated that, is beside the point. Commented Apr 13, 2018 at 15:07
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    . . . . . and diseases of Venus are "venerial diseases". And some saints are accorded "veneration". Commented Apr 13, 2018 at 21:00
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    This is a question about Latin, not English.
    – David
    Commented Jun 26, 2020 at 18:56

1 Answer 1


English has borrowed many words from Latin and often the plural in the original gets carried along, instead of just using the English plural.

  • usually words ending in -us (second declension) have a plural in -i, and that is the most common 'true-to-latin' plural in English is: eg alumnus -> alumni (though pronounced English style: uh-LUM-nigh, different from the more Latin ah-LUM-knee).
  • but the plural, in Latin, for some Latin neuter nouns ending in -us is different because they fall under the third declension, with its own set of endings. -a is the nominative plural ending for third declension neuters), and under which the final -s undergoes the Latin plural sound change called rhoticization changing s to r: opus -> opera, which means work -> works, an opera is originally a collection of pieces. This is the answer to why the -Latin- plural has 'r' instead of 's', because it was in the transition in Proto-Italic between Old and Classical Latin when the rhotacization occurred.

Opus/opera, genus/genera, corpus/corpora are the relatively more common examples, the others are much rarer (viscus/viscera).

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    It isn't an irregular plural, it's just different from the plural for second-declension -us nouns. For third-declension nouns, stem+<a> is the regular plural for neuter nouns, including the ones ending in -us. The stem itself, it's true, needs to be learned verb by verb for the third declension, though there are patterns. Rex/reg-, nox/noct-, mens/ment-, ops/op-, opus/oper-, etc. (I know you know these details, but I'm presenting them for others to explain my point about this being a regular construction.) Commented Apr 13, 2018 at 14:55
  • @GreenGrassoHolm Frankly, anything to me that's not first or second declension is irregular. What a mess. You want me to learn five cases, singular and plural, and for five different declensions? What, locative too? And neuter_ forms? Shut the front porta.
    – Mitch
    Commented Apr 13, 2018 at 15:33
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    or portam. Whatevs
    – Mitch
    Commented Apr 13, 2018 at 15:34
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    Your complaint about the complexity of Latin is duly noted. :-) Do yourself a favor and avoid Icelandic. :-D The back of my Icelandic grammar presents 74 different noun paradigms, and that's before getting into irregular nouns. Commented Apr 13, 2018 at 15:36
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    You may also want to check out this page: vocabulary.com/articles/wc/a-strange-plural-phenomena
    – Adamant
    Commented Jun 26, 2020 at 17:17

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