Most discussions I have read on the issue of Latin plurals in English only address words ending in -us, -um, -x and the feminine nouns ending in -a. I would like to know how to make the English plural of Latin masculine nouns ending in -o, such as "folio". I am tempted to say and write "folia" as the plural of "folio", and similarly "bifolia" as the plural of "bifolio" etc, although my Oxford Advanced Learner's only quotes "folios". I am sure I have read both versions of this plural in academic print before. What would you advise?

  • In what sense are you using the word?
    – herisson
    Sep 15, 2016 at 2:33
  • 1
    Merriam-Webster has given their own view on how to form plurals with Latin-derived nouns (paraphrasing): "When a word from a foreign language is taken into English, forming the plural as you would with other English nouns becomes fair game; the word is, after all, now an English word. Hence the plural of octopus is octopuses (but you can use octopi or (more appropriately) octopodes if you want to)."
    – scottb
    Sep 15, 2016 at 5:28

3 Answers 3


I would advise using the only plural form that you found listed in your dictionary (and in general, following the advice given by arnsholt in an answer to a related question: "Unless you are absolutely, completely sure you know the correct classical plural, or the classical plural is the normal plural, use the English plural").

The Online Etymology Dictionary explains that this word is derived from the ablative singular form of the Latin word folium "leaf, page." It does not come from a Latin noun that ends in "o" in the nominative singular.

That said, there are words like that (such as ratio; this is actually feminine in Latin, but it is in the third declension so it doesn't really affect how you pluralize it). Many of them have "n" in their other forms in Latin (the Latin plural of ratio is rationes). I've never heard or seen anyone use plurals like "rationes" in English. In my experience, people always use "ratios." (In Grammar of the Latin Language, by Leonhard Schmitz, I found a masculine example: pugio "knife," plural in Latin pugiones.)

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    Most hominids can think of a third-declension masculine ending in -o. :)
    – tchrist
    Sep 15, 2016 at 2:42
  • @tchrist: well, I was trying to think of a Latin word ending in "-o" that has become naturalized in English.
    – herisson
    Sep 15, 2016 at 2:45
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    I’m thinking homo probably doesn’t fit that bill. :)
    – tchrist
    Sep 15, 2016 at 2:46
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    Arguably, if you wanted to use the Latinate plural, it should be foliis. But such forms are follies. Sep 15, 2016 at 11:44
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    Older law books talk about rationes decidendi but there "ratio" is a Latin word meaning "reason". Sep 15, 2016 at 12:33

The Latin word was folium, a neuter singular whose plural was folia. The ablative case was folio, and that is what the English word is borrowed from during the Late Latin period.

There is no reason not to use the regular English plural, folios. I have never seen anything else except for purely Latin formulations like folia botanica, which is clearly meant to be Latin not English.


Since folio itself is not a Latin word, there is no Latin plural. I would say that folio is Anglicized Italian. At any rate, it is English, so the plural is simply folios.

  • "Folio" is a Latin word; it's just not in the nominative.
    – jwodder
    Sep 15, 2016 at 13:43
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    @jwodder: actually, the Oxford English Dictionary says it may also be (at least in part) from Italian "foglio." Oxford Dictionaries also mentions "foglio" as a possible source.
    – herisson
    Sep 15, 2016 at 14:52
  • Whether or not Italian usage influenced the English borrowing doesn't change the fact that folio is a Latin word.
    – chepner
    Sep 15, 2016 at 18:36
  • @chepner: depends on what you mean by "word," I suppose. It is a word-form that could be found in a Latin text, but it is not a Latin lemma (it wouldn't be listed as a headword in a Latin dictionary).
    – herisson
    Sep 15, 2016 at 23:26
  • @suməlic: I'm not sure why that's relevant. If a creole has a word "wen" derived from English went, would you argue that went is "not an English word" just because English lexicographers have selected the bare infinitive, in this case go, as the lemma?
    – ruakh
    Sep 16, 2016 at 0:12

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