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I've never been certain of the rules surrounding the use of the -i suffix for pluralizing a word. I had thought that it was used for any word whose singular ended in an 's', but that doesn't appear to always be true.

For example, the plural of octopus can be written octopi. But the plural of chorus seems to always be choruses, never chori.

When is it proper to use an -i for pluralization?

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    The short answer is "only if you've seen the plural formed using -i by a respected source". In all other cases, make your plural by adding -s. If you happen to be picked up on this now and then, you can reasonably suggest you're just "ahead of your time", since it's highly likely all plurals will eventually be regularised. – FumbleFingers Feb 21 '12 at 16:28
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    @FumbleFingers Agreed, but I’d modify what you said to say either -s or -es depending on the regular rules of English plurals. – tchrist♦ Feb 21 '12 at 17:50
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    The plural of 'octopus' is 'octopuses' or 'octopodes,' not 'octopi.' – Robert S. Feb 21 '12 at 17:56
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    @RobertS. dictionary.reference.com/browse/octopus -- -i pluralization is accepted by some. But regardless: you get the idea behind the question. – Ian C. Feb 21 '12 at 18:20
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    @FumbleFingers I have a whole theory about that. We use -s/-z/-ɨz for either of the plural or possessive inflection, but not both. If it would be doubled, we surprise it. Also -iːz words are left alone. Hence 2 series’ ends,James’s pal,Mr Jones’ shirt,the Joneses live at the the Joneses’ farm,these series’ starts,Diogenes’ lamp etc. It’s ’cause folks are hung up writing instead of speaking that they are always getting these things spelled wrong, which is so strange because they by definition never say them wrong. But they think spelling trumps speaking, letting the tail wag the dog. – tchrist♦ Feb 21 '12 at 20:58
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Use "-i" for plurals when the word is borrowed from a Latin word that used "-i" for plurals. Note that in Latin not all words that end in "-us" are made plural by changing the "us" to "i". For example, in Latin the plural of "locus" (place) is "loci", but the plural of "fructus" (fruit) is "fructus".

Octopus comes from Greek and not Latin, and so does not follow the Latin rules.

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    Well, the octopuses octopi octopodes debate is gettting to be a bit of a hoary old chestnut, but just a reminder that - depending on your chosen "authority", any or all can be considered "correct" – FumbleFingers Feb 21 '12 at 16:24
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    “Octopus” is still a bona fide Latin word, just not from a declension which has a plural ending on -i (or even that … see Fumble’s excellent link). – Konrad Rudolph Feb 21 '12 at 17:08
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    @tchrist Are you saying that a word could be made plural by adding "-i" even if it doesn't come from Latin? Sure, I suppose my answer is poorly worded in that sense. Latin isn't the only possible source of "-i" plurals, just one source. Surely the most common source in English, but not necessarily unique. – Jay Feb 21 '12 at 18:38
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    Yes, that’s what I was saying. I voted for you anyway. :) – tchrist♦ Feb 21 '12 at 18:41
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    @Jay “oktapous” (well, ὀκτάπους) is a Greek word. “octopus” is its Latinisation. I’m not actually sure that it was used in the Roman Empire but that’s irrelevant here since its Latinisation happened (according to the video in the comment above) in England in the 17th century. Which plural to use is a pure matter of convention, since there are no actual binding rules defining how grammar works (and if there are, they always have exceptions). – Konrad Rudolph Feb 21 '12 at 18:48
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I've never been certain of the rules surrounding the use of the -i suffix for pluralizing a word.

There are many nouns whose plurals end in -i, and contrary to popular misconception, quite a few of them did not get there from a -us singular.

Here from the OED is a semi-random assortment of nouns whose plurals end in -i, with their singulars included:

abacus > abaci, agape > agapai, albergo > alberghi, alumnus > alumni, aptychus > aptychi, bacillus > bacilli, bajocco > bajocchi, bandit > banditti, blin > blini, caduceus > caducei, calzone > calzoni, capo > capi, castrato > castrati, casus belli > casūs belli, centumvir > centumviri, cognoscente > cognoscenti, concerto grosso > concerti grossi, conoscente > conoscenti, conversazione > conversazioni, denarius > denarii, dilettante > dilettanti, diplococcus > diplococci, divertimento > divertimenti, duumvir > duumviri, frate > frati, fungo porcino > funghi porchini, fungus > fungi, genius > genii, glissando > glissandi, graffito > graffiti, humerus > humeri, ichthyosaurus > ichthyosauri, intaglio > intagli, intermezzo > intermezzi, isthmus > isthmi, kernos > kernoi, laika > laiki, leu > lei, libretto > libretti, maestro > maestri, mafioso > mafiosi, Magus > Magi, miles gloriosus > milites gloriosi, modello > modelli, modulus > moduli, modus > modi, mondo > mondi, monsignor > monsignori, niello > nielli, nucleus > nuclei, obelus > obeli, oboe d’amore > oboi d’amore , onager > onagri, paparazzo > paparazzi, papyrus > papyri, phallos > phalloi, phallus > phalli, ragazzo > ragazzi, ranunculus > ranunculi, ricercar > ricercari, ritardando > ritardandi, samurai > samurai, Scomber > Scombri, scudo > scudi, septemvir > septemviri, sestiere > sestieri, sforzato > sforzati, siglos > sigli, siglos > sigloi, squadrist > squadristi, squalus > squali, strategos > strategoi, strategus > strategi, suggestio falsi > suggestiones falsi, syllabus > syllabi, talus > tali, tarantato > tarantati, tempo > tempi, terminus > termini, thalamos > thalamoi, thalamus > thalami, thesaurus > thesauri, thymus > thymi, topos > topoi, torus > tori, triumvir > triumviri, udarnik > udarniki, umbilicus > umbilici, uomo universale > uomini universali, uomo > uomini, uterus > uteri, vagus > vagi, vaporetto > vaporetti, viale > viali, volcanello > volcanelli, zeppole > zeppoli, and zita > ziti.

Similarly, there are plenty of -us nouns that do not go to -i plurals. Many of these are from the Latin 3rd declension, and some are from its 4th. Others aren’t from Latin nominative singulars at all, like onmibus, a dative plural, and ignoramus, which was a verb in the 1st person plural present indicative active (the Romans were really tense, you know).

Here are a few from the OED:

apparatus > apparatus, callus > calluses, cantus > cantus, chorus > choruses corpus > corpora, crus > crura, genus > genera, glomus > glomera, hiatus > hiatus, hippopotamus > hippopotamuses, ignoramus > ignoramus, isthmus > isthmuses, logodaedalus > logodaedale, magnum opus > magna opera, meatus > meatus, nautilus > nautiluses, octopus > octopodes, omnibus > omnibuses, pectus > pectora, planctus > planctus, plexus > plexus, prospectus > prospectus, sinus > sinuses, status > status, status > statuses, subgenus > subgenera, summum genus > summa genera, urus > uruses, and Venus > Veneres.

Hm, I’d thought Venus was a rare 2nd declension feminine like humus. Guess not.

As you can see, the only reasonable answer is to look it up.

(Insert mumbles about hapax legomenon > hapax legomena.)

EDIT

And before it comes up, the plural of virus is simply viruses in English.

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    +1. Take-home point: there are no good general rules (aside from "use whatever's standard"); just consult a dictionary whose guidance you trust. – ruakh Feb 21 '12 at 18:11
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    Actually, as many dictionarii as you can. – Kris Feb 22 '12 at 7:27
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Among other reasons it depends on the root being Greek or Latin, however there are many reasons for the ending being us/i as seen in this wikipedia entry which in turn is heavily discussed in the rest of this post, so I recommend you take the Wikipedia entry cum grano salis/άλας ;)

http://en.wikipedia.org/wiki/Octopus#Etymology_and_pluralization

The term "octopus" is from Greek ὀκτάπους[37][38] (oktapous, "eight-footed"), with traditional plural forms "octopuses" (pronounced /ˈɒktəpʊsɪz/) from English grammar and "octopodes" (pronounced /ɒkˈtɒpədiːz/) from the Greek. Currently, "octopuses" is the most common form in both the US and the UK.

Some authorities find that octopi is an objectionable hypercorrection, feeling that the form arose from the incorrect assumption that "octopus" is a Latin 2nd declension form.

However, "octopus" is a Scientific Latin 3rd declension noun with a plural of octopodes. Nevertheless, the Oxford English Dictionary lists "octopuses", "octopi", and "octopodes" (in that order), labelling "octopodes" 'rare' and noting that "octopi" derives from the misapprehension that octōpus is a second declension Latin noun.

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  • Is there a way to determine a word's root as being Greek or Latin? – Ian C. Feb 21 '12 at 16:03
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    Etymology dictionary... But most dictionaries will have the plural after the word – mplungjan Feb 21 '12 at 16:06
  • Several. One can learn Greek or Latin, for instance. Or one can look up the word in a dictionary or a book of Greek and Latin roots (there are a number). Further information available here. – John Lawler Feb 21 '12 at 16:07
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    @IanC. Actually, it’s a great deal more complicated than this. You can’t just claim Latin vs Greek. That’s so oversimplified as to be wrong. – tchrist♦ Feb 21 '12 at 17:47
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    I’ll bet you that no Roman ever uttered the word octopus, let alone made it a plural. It was coined by Linnaeus during the 18th century. I’d like to see the source the pretends that it was from the 3rd declension, whose -us words ended in -ora or -era in the plural. There were Greek irregulars thrown around the Latin declensions, but this is one the Romans never used. – tchrist♦ Feb 21 '12 at 18:15

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