This post first appeared on LinkedIn in 2022 and has been lightly edited for this site.
Recently, I wrote about awkward plurals used in English, focusing mostly on the linguistic history related to why we say the things we do. Today’s post is a touch different. While there is some information about etymology, there are several other practical tips that can be used daily with plurals derived from Latin.
Definitions
The nuts and bolts of getting on with today’s topic rely on knowing a bit about Latin declensions. But since most people weren’t subjected to the horrors I faced in secondary school, I’ll go easy. First off, declensions are simply classifications of nouns that share certain inflections. In Latin, there are five, and each one tells us how to build a plural: -ae, -i, -a, -es, -us, or -ua (Green 2015, 30–31). Yes, that’s six endings for five declensions. Gender is the culprit—second- and third-declension neuters take -a rather than -i or -es, and fourth-declension neuters take -ua rather than -us—and the third and fifth declensions both land on -es. The real kicker, however, is that the application of these inflections in English appears random, and for good reason: it often is (Fowler and Burchfield 2000, 442). Don’t worry about memorizing any of that.
Come with Us
Making plurals from words ending with -us can be controversial for several reasons. The main one comes from differing opinions of what is “proper.” For example, Latin second-declension words ending in -us can generally be inflected with -i to make a plural form—alumnus becomes alumni; cactus, cacti; fungus, fungi; and locus, loci (Fowler and Burchfield 2000, 815). While words from this classification can have the -i plural inflection in English, many, but not all, can also take on the -es ending, so we can have secondary English plurals like funguses and cactuses. However, locus cannot be locuses, nor can alumnus be alumnuses. There are few reliable rules with this class of nouns, so it is important to check with your preferred dictionary and style guide to ensure you get it right.
With Latin fourth-declension words, the plural form of -us words in English is rarely anything but -es, and certainly never -i in ordinary English usage. So, we end up with words like prospectuses, apparatuses, sinuses, and others (Fowler and Burchfield 2000, 815). Status belongs to this group too, which is why stati isn’t a thing.
Third-declension Latin loanwords ending in -us include corpus, genus, and opus. And these are made plural with -a, giving us corpora, genera, and opera, respectively. With these, -es is almost never used. I think I can see you eyeing opera and wondering what is going on there since we have operas. Good catch. This one is a bit awkward. Like several other words, this one came into English twice—first by way of Italian in the 1640s, arriving as a singular, and the second isn’t known, but it wasn’t recorded until 1809. Yes, the plural of opus, meaning “work” and often referring to an artistic work, is opera, and an alternative plural is opuses, which falls into the “almost never” category just mentioned. At the same time, we have opera as a singular noun meaning “a drama set to music,” and it is made plural by adding -s. If anything, one can walk away knowing that opus, opera, opuses, and operas demonstrate that English is convoluted, and often unnecessarily so.
It’s All Greek to Me
The big problem with -us words, as if there weren’t enough problems already, comes from words with Greek roots being treated as those with Latin roots. An often-cited example is octopus. For the record, the inflected -es plural is generally the preferred form. Fowler and Burchfield (2000, 539) are pretty clear: “The only acceptable [plural] in English is octopuses.” Garner (2016, 645) supports this opinion explicitly. The issue with using the plural octopi is that the -i inflection belongs to the Latin second declension, and octopus doesn’t. It comes from the Greek oktōpous (stem oktōpod-), a third-declension noun, so its -us is not the same ending found in alumnus or fungus. Greek origin alone isn’t the issue: cactus comes from the Greek kaktos, which Latin absorbed as a regular second-declension noun, so cacti is legitimate. Octopus, by contrast, is an eighteenth-century New Latin coinage that never fit that pattern. Merriam-Webster lists octopi and octopodes as alternatives—the first from a declension octopus never belonged to, the second usually deployed for comedic effect. Neither is worth the trouble. Write octopuses.
It is a good idea to exercise the same general approach with rhinoceros, which also comes from a Greek third-declension noun (rhinokerōs, stem rhinokerōt-). Yes, I know that rhinoceros ends with -os, but there is still a propensity toward making the plural rhinoceri for some reason, so I’m throwing it into the same bucket. Just write rhinoceroses. Hippopotamus is different. It comes from the Greek hippopotamos, which Latin treated as a regular second-declension noun, so hippopotami is as legitimate as cacti. Hippopotamuses is also correct, so check your preferred dictionary and style guide.
Conclusion
You are probably asking yourself how you could possibly know what these Latin declensions are. There are two ways to approach it. The long way is to take Latin or look in a Latin dictionary. The short way is to ignore the rules and rely on English dictionaries and spell check.
I’m skipping out on -um nouns today. In a previous article, I went over the difference between datum and data. The same rules generally apply to other -um nouns. Some of their Latin plurals have even become English singulars: agenda was originally the plural of agendum, so yes, you can have an agenda and several agendas. I may do another article specifically on those and a few others.
References
Fowler, H. W., and R. W. Burchfield. 2000. The New Fowler’s Modern English Usage. 3rd ed. New York: Oxford University Press.
Garner, Bryan A. 2016. Garner’s Modern English Usage. 4th ed. New York: Oxford University Press.
Green, Tamara M. 2015. The Greek & Latin Roots of English. 5th ed. Lanham, MD: Rowman & Littlefield.