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We’ve all had that moment of indecision when pluralizing a Latin-derived noun: should I use the traditional Latin plural and risk seeming snobbish, or should I use the anglicized plural and risk seeming uncultured? However, there’s an even bigger pitfall, which this blog post might help you avoid.

Before we address that, some purists believe the Latin plural is always correct, so the English plural is always wrong: that extreme view rejects linguistic change, and it implies Latin-to-English imports like “senator” and “tractor” should use the Latin plurals senatores and tractores. At the other extreme, some nativists believe the English plural is always preferable, while the Latin plural is an irrelevant relic, but this is wrong too: if you referred to the *”alumnuses” and *”alumnas” at your high school reunion, the school would probably revoke your diploma. The moderate course of action is to consider whether the English or Latin plural is more frequently used, consider your audience, and consider the linguistic spin you wish to impart.

The number one thing to avoid, however, is using an incorrect Latin plural.  No matter how Latinate they may sound, and no matter how correct (or hypercorrect) you’re trying to be, incorrect plurals are foolish.  Consider these words that might seem like second declension masculine nouns, with singular -us changing to plural -i, but aren’t.

1. Caucus – It’s easy to imagine Roman senators adjourning into smaller groups called *”cauci.”  However, that’s wrong because “caucus” isn’t Latin-derived at all: it’s of Native American (Algonquin) origin, so stick with “caucuses.”

2. Ignoramus – This is from Latin, but the plural *”ignorami” is wrong because “ignoramus” comes from a verb, not a noun. It means “we are ignorant” or “we do not know” and entered English as a jury verdict between innocent and guilty, likely gaining its current English meaning through the title of a 1615 legal satire. To avoid linguistic guilt, say “ignoramuses” instead.

3. Virus – We’re getting closer because this is a Latin noun—but it’s neuter, and plurals of Latin neuter nouns always end in -a.  In Latin, the word virus was used only in the singular, but if we had to construct a plural, it would be vira.  The Romans didn’t discover the germ theory of disease, so the Latin word virus meant “poison” or any other harmful element.  *Viri is wrong as a plural because the -us would change to -i only if it were a masculine second declension noun; oddly enough, virus is the only neuter second declension noun in Latin. (Viri is a common Latin word as the plural of vir, meaning “man,” which is related to “virility” but not “virus.”) Given that classical Latin didn’t pluralize this word, and no English speaker says vira, the healthy choice is to stick with “viruses.”

4. Status – If you’re discussing the statuses of various projects, you might wonder if it should be *”stati”, but it shouldn’t.  “Status” is taken directly from Latin, and it’s a masculine noun, but it’s not second declension: it’s fourth declension. By fourth declension rules, the Latin plural of status is…status.  (The plural, unlike the singular, is pronounced with a long U, so sometimes it’s written statūs with a macron over the U.)  Latin fourth and fifth declension nouns in the nominative case have the same spelling in singular and plural.  Using “status” as a plural would confuse English readers, so you’re better off with “statuses.”

5. Octopus – The correct plural of “octopus” has become a classic gotcha, as many people are quick to say that “octopi” is wrong because “octopus” derives from Greek, not Latin, and therefore the authentic plural is octopodes.  That’s the right end result but not a complete explanation because octopus is also the Latin word for “octopus,” so we need to examine the chronology.  The Latin word octopus is a modern scientific coinage, first cited by Linnaeus in 1758, which is also the year in which it is considered to have entered English—so the English word really is derived from a (modern) Latin word. (In classical Latin, this creature was called a polypus, a Greek-derived term meaning “many-footed” rather than the more specific “eight-footed.”)  Admittedly, the modern Latin word derives from Greek, but so do many classical Latin words.  In light of all that, why isn’t *”octopi” the Latin plural?  Because when Linnaeus coined the modern Latin word, he retained the Greek stem by making it a third declension noun (octopus, octopodis), which therefore makes the nominative plural octopodes, a perfectly valid Latin plural. The classical Greek singular was ὀκτώπους (oktopous), and the classical Greek plural was ὀκτώποδες (oktopodes, plausibly transliterated as “octopodes”). Given the three choices found in most English dictionaries, I avoid “octopodes” (possibly confusing and possibly pretentious) and “octopi” (which has nothing to recommend it) in favor of the neutral “octopuses.”

6. Genus, corpus, opus – These nouns don’t change -us to -i because they’re neuter third declension, not masculine second declension. As mentioned above, all neuter nouns (regardless of declension) end in -a, but third declension nouns have an added twist because their stems often (but not always) add one extra syllable that appears everywhere except the nominative singular. So *”geni,” *”corpi,” and *”opi” are unquestionably wrong: the correct Latin plurals are genera, corpora, and opera.  Unlike some of the Latin plurals given above, “genera,” “corpora,” and “opera” are all commonly used in English.  You might sometimes hear people mention “corpuses” rather than “corpora,” but the other two Latin plurals are almost obligatory in English because biologists don’t usually say “genuses,” and *”opuses” sounds silly unless you’re referring to cartoon penguins.

These aren’t the only unusual -us nouns, but they illustrate the primary ways a -us noun can defy an -i plural.