When should I replace an 'us' with an 'i'?

Are the following words valid?

Hippopotami (plural of hippopotamus)

Virii (plural of virus)

Bonii (plural of bonus)

When should I use 'i' and when should I just add es? Where does the i come from? And, when should I use 2 i's and when should I use just one? Thanks.

  • And radius: radii. – M.A.R. Feb 17 '15 at 5:35
  • Cactus is Cacti :P – Catija Feb 17 '15 at 5:36
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    Some of these are going to be regional. Out in West USA, ordinary folks are gonna switch between cacti and cactuses. – user6951 Feb 17 '15 at 13:13
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    As an aside, the word bonii is definitely incorrect. Use bonuses – Devon Parsons Feb 17 '15 at 15:02
  • Don't forget octopuses ("octopi", octopodes) – March Ho Feb 18 '15 at 22:13

According to Oxford Dictionaries online,, "hippopotamuses" is the preferred plural. This article on oxforddictionaries.com deprecates "hippopotami" . (It also discusses "cactus" and a few other "-us" words.)

Other dictionaries, such as Dictionary.com, offer both "hippopotamuses" and "hippopotami" as acceptable alternatives. (It depends on whether you consider it Latin-ish or Greek-ish)

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    I "attached" the link to "this article". Hope you don't mind. – M.A.R. Feb 17 '15 at 9:58
  • Thanks. I wish I knew how to do that. On IPhone there is no right-click. – Brian Hitchcock Feb 17 '15 at 10:01
  • @Snailboat: my bad. Removed mention of OED. – Brian Hitchcock Feb 17 '15 at 10:19
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    Please, never let yourself be cowed by "deprecations" made by the drudges who write dictionaries. Language is to be used. Use hippopotami when you want its effects and hippopotamuses when you want its effects. (To be fair, I should mention that the article wisely does not actually deprecate "hippopotami", it says it sounds funny and pedantic. Those are effects you can exploit.) – Ben Kovitz Feb 19 '15 at 6:51

Hippopotamus is a Latin word that has been imported into English. In Latin, its plural is hippopotami. In English, we sometimes still use the Latin plural. Like many unusual and interesting English words, it was first coined in Greek and later absorbed into Latin. In Greek, it was hippopótamos, with plural hippopótamoi.

How to think about this

Latin and Greek plurals are primarily jargon used in scientific or academic work. For example, antennae, the Latin plural of antenna, is preferred in entomology, but the English plural antennas is more common in popular and informal writing.

However, many Latin and Greek plurals also have popular usage: basis/bases, memorandum/memoranda, and many others. These often carry a feeling that they are not fully native words: we're "borrowing" them from Latin and Greek. The more the word seems like a fully adopted member of the English language, the more likely we are to give it an English plural. For example, as a plural of index, the English-style indexes has a long-running competition with the Latin indices. In some cases, like basis, very few people use a native plural, maybe because it would sound too weird (basises—hmm, sounds a little like hippopotamuses).

To fully understand this, you have to know the basics of the history.

The history

From about 800 to 1800 C.E., Latin was the international language of Europe and the main language of science, law, administration, scholarship, and religion. Latin had several advantages that made it well suited for these roles. It was nobody's native language, so no one country "owned" it. If you went to a university, you learned Latin, and you spoke in Latin at the university, at least during classes and lectures—even in your native country. Latin was defined by a written tradition going all the way back to ancient Rome. This made Latin remarkably stable. While its vocabulary grew, its core changed very little over the centuries—much less than vernacular languages that people learn as their mother tongue.

Latin words came into English in two ways. One way was through the conquest of England in 1066 by the French-speaking Normans. French grew out of Latin, and English absorbed a huge number of words from it. The other way was by borrowing directly from Latin when you needed precise, specialized jargon not subject to the haziness and fluctuations of everyday language. If Latin didn't have a word, you borrowed it from Greek. Today, the majority of English words come from Latin (or Greek, usually via Latin).

The period of the rapid expansion of English vocabulary is long past, as is the tradition of most educated people learning Latin and Greek. However, we still exploit Latin's stability and internationality in the scientific names for species and in a lot of scientific, academic, and legal jargon. And we still retain the tradition of indicating word origins in spelling—and a little bit in grammar. (We even have a few Hebrew and Italian plurals in English.)

The plurals

Latin plurals are more complicated than native English plurals. Latin nouns fall into several different patterns, called declensions. Hippopotamus follows the "second declension", like stimulus/stimuli, nucleus/nuclei, fungus/fungi, and others. But corpus and genus are "third declension", with plurals corpora and genera. And apparatus is "fourth declension", with plural apparatus. Some "first declension" Latin nouns in English are larva/larvae and differentia/differentiae. The other main patterns are quantum/quanta, curriculum/curricula, bacterium/bacteria, and appendix/appendices, vertex/vertices, matrix/matrices.

I've been searching for the past 15 minutes and I still haven't found a good source of information about these for an English learner, but this might be helpful. You don't need to know which declension is which. You get familiar with each plural after you run into it a few times, and that's good enough.

The -ii plural ending only comes up when the singular ends in -ius, as in radius/radii and denarius/denarii (which are the only two I can find).

The plural of virus is viruses and the plural of bonus is bonuses. These words do not have Latin plurals in English (although people sometimes enjoy pretending that they do). If you'd like to know why they don't, that's probably a good question for ELU. A short answer that skips many details is: viri is Latin for "men", not "viruses"; and boni in Latin means "good men", not "bonuses".

How to choose

As shown on this graph, hippopotami was much more common than hippopotamuses for most of the last 300 years, but today they're about equally common. Some people would say that this proves scientifically that both are now equally "correct" or "acceptable". I hope that you won't think that way. I consider that pseudo-science. When using a Latin plural, you should simply be aware that that's what you're doing. As you find Latin plurals used, or not used, in different situations, you should develop some feeling for how different people in different situations perceive them.

You should get a sense that the word hippopotami is fun to say and sounds a little pedantic, but hippopotamuses is also fun to say and can sound a little silly since a six-syllable word ending in -us seems to cry for a Latin plural and the unstressed -amuses is unusual (amusing, even). You should choose which form to use depending on the effect you want to produce in other people, the situation, your mood at the moment, your audience, and any other reason you like—exactly as you should choose any other word. But you're a zebra, so you already know all that.

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    I am indeed a zebra. And I like making things. – Pyraminx Feb 19 '15 at 2:24

-us to -i is part of Latin root words.

Not all words get pluralized that way, though and many are pluralized using the standard rule "add an -s or -es". For Hippopotamus, you have the choice of:

  • Hippopotami
  • Hippopotamuses
  • Hippopotamus (can also be used as plural)

Different sources prefer one over the other, though there seems to be a lean towards hippopotamuses.

Personally, if I'm not doing it for some formal writing, I just go with hippos.

Also, there's a good Wikipedia article about this topic: Plural form of words ending in -us... and the plural of virus is viruses and for bonus it's bonuses.

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    Just a note that it's not true that all Latin words ending in -us have plurals in -i. Words like status or nexus, which belong to the fourth declension in Latin, in modern English take native English plural endings (i.e. statuses, nexuses). – jogloran Feb 17 '15 at 10:22

The important thing to note here is that us native English speaker have no idea what the plural of Hippopotamus is, either. Hippopotamouses? Hippopotamice?

Generally, we stick with "Hippos", or say "Hippopotami? Hippopotamuses?" and then have a fun 5 minutes where everyone nearby discusses it.

The simple answer is that most people don't know, or care enough, to pull you up on it if you're wrong. But if in doubt, go with Hippos, it's common enough not to be informal, and would be acceptable in any circumstance apart from perhaps serious Hippo research establishments.

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  • actually, that's a valid approach, brcause hippo means horse, and potamus means "of the river", so it makes sense to pluralize the first part rather than the last. I've seem some discussion that suggested (tongue-in-cheek) "hippoepotamus" which is how the Greek plural would be formed. And plural "octopus" would be "octopode" – Brian Hitchcock Feb 17 '15 at 10:21
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    My personal favourite is Hippopotamice. Because it also sounds like a superhero team. – Jon Story Feb 17 '15 at 11:07

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