The editors were discussing a story about the health benefits of a particular type of cactus, and maybe others. The story was called “Cacti.” “Is it cactuses or cacti?” one asked. “The plural of cactus is cactuses,” another said, adding wryly: “Is the plural of circus circi?”

Yes, another Latinate ending that made it into some English words, but not others. And, sad to say, there’s no easy trick to which ones are pluralized by changing the “-us” to an “i” and which ones get the “es” treatment.

For some of the words, there is no debate. The plural of “circus” is “circuses,” not “circi.” The plural of “alumnus” is “alumni,” not “alumnuses.” The plural of “campus” is “campuses,” not “campi.”

But the “cactus” question is a prickly one. Dictionaries differ on which plural they prefer, though they list both. Webster’s New World College Dictionary likes “cactuses,” while American Heritage and Merriam-Webster go with “cacti.” Therefore either is correct. (M-W says “cactus” is also an acceptable plural.) Your choice may also depend on context: If you are speaking of the plants from a botanist’s view, “cacti” is preferred.

Another disarming “-us” word is “octopus.” Its plural could be “octopuses” or “octopi.” Most dictionaries prefer “octopuses”; some linguists sniff at “octopi” as “hypercorrection,” used by people who bend so far backward trying to be faithful to supposed rules of grammar that they fall over. “Octopi,” though, is quite common in print: It has made dozens of appearances in The New York Times, for example, even though The Times stylebook wants “octopuses.”

Other plural forms of “octopus” that have been used are “octopod” or “octopodes.” An “octopod” is any creature with eight limbs, including an “octopus,” so that won’t fly. Showing how language evolves, Webster’s New International Dictionary, which ruled from 1909 to 1961, listed “octopodes” as second choice for plural. Nowadays a few dictionaries still list “octopodes” with “octopus,” but as a variant and singular. The plurals, they say, are (in order of preference) our friends “octopuses” and “octopi,” and also “octopodes.”

In a multipage entry on plurals, Garner’s Modern American Usage seems to throw up its metaphorical hands on how to pluralize “-us” words. In essence, it says that if a word “imported” from another language has been “naturalized,” it takes the plain-vanilla plural (“s” for most words, “es” for most ending in “s” or “z”). “But if a word of Latin or Greek origin is relatively rare in English—or if the foreign plural became established in English long ago—then it typically takes its foreign plural.”

So who determines whether a word is “relatively rare” or “became established in English long ago”? Why, you do, of course. How people use words determines whether they are “correct.” The more people use them, the more “correct” they become.

Then there is “virus.” When it started taking hold as the description for a software program that makes a computer do things it shouldn’t, some programmers, known for having fun in the naming of new things, apparently decided that the plural “viruses” was boring. Instead, for a few years into the mid-2000s, the plural was occasionally “viri” or “virii.” Thankfully, that disease seems to have run its course, and “viruses” prevails, except in mocking contexts.

 

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years.