Wikipedia lists some rules and suggests that the plural form of olio is olii.
I used to think that oli sounds (and writes) better, and Dizionario d'ortografia e di pronunzia confirms that.
Treccani, however, suggests yet another plural form, with circumflex: olî.

Any rule of thumb I could use?


We can derive a general rule (with loads of exceptions, like everything in this language) from this case. When the expected plural form of a word contains a non-stressed double i, like "olio" -> "olii", "principio" -> "principii", there's a tendency to drop the second i: "oli", "principi" (Ngrams for "olii" vs "oli" confirm this graphically). However, some speakers do retain a memory of the double i in their pronunciation of such words, the i is slightly longer.

To tell a "single-i-that-used-to-be-double" from an ordinary single i the circumflex accent can be used and this is standard Italian grammar. This makes sense particularly when one needs to disambiguate, for example "principî" (plural of "principio", principle) as opposed to "principi" (plural of "principe", prince); this is the solution used in the Italian Constitution that includes a chapter called Principî fondamentali. Another solution in this case is the tonic accent: "principî" can be unambiguously written as "princìpi" because the other word is read as "prìncipi".

Conclusion: the three forms ("olii", "oli" and "olî") are equally acceptable, but as you can imagine the circumflex accent is unusual and most people wouldn't even be able to type it on their keyboard. AFAIK, this is the only case in which a circumflex accent can be used in Italian words not borrowed from foreign languages or dialects, I guess most Italians wouldn't even realise that in their language this accent exists.

Notice that the situation is entirely different with stressed plurals in -ii (e.g. "sgocciolìo" -> "sgocciolii"). In this case the double i is pronounced clearly by all speakers (the first i is stressed) and is compulsory. The opposite applies to nouns ending in -cio, -glio, -scio etc. where the i is not really used as a vowel: the plural form requires one i.

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    You forget to mention that the letter j was often used as a second i: accademiadellacrusca.it/it/lingua-italiana/… – Sklivvz Dec 27 '13 at 10:47
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    @Sklivvz Such usage of j is definitely obsolete though. – Mauro Vanetti Jan 15 '14 at 14:10
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    Not more than the circumflex accent, in my opinion. – Sklivvz Jan 15 '14 at 14:19
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    @Sklivvz The circumflex accent is standard 2014 Italian, even though it's seldom used. The j is just obsolete. I gave the example of the Constitution, it contains the circumflex but it does not contain the j. The Crusca article you quoted states precisely this: the circumflex accent is OK, the j is not. – Mauro Vanetti Jan 15 '14 at 14:26
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    @Sklivvz 21st-century Italian does not use 18th-century Manzoni's Italian as a reference for spelling rules. This language has changed in the last centuries much faster than English. However, I just checked the full text of "I promessi sposi" and Manzoni does not use the j. This letter is only used for plurals when Manzoni quotes 16th-century "grida". Bottom line: the j sounded ancient and obsolete already at Manzoni's ears! :-) – Mauro Vanetti Jan 15 '14 at 15:01

Using î instead of ii is something that was once done, but nowadays that is not done anymore, at least on everyday usage. Between olii and oli, I would expect the latter to be used more often than the former, at least because it requires you to write less letters, and generally there is no confusion about what oli means. (It would not be taken as plural of olo, since that is not an Italian word.)

Lo Zingarelli reports just two plural word for olio: oli and olii.

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    There is a long list of words for which the plurals in -i and in -ii (or ) belong to different words, from assassini(i) to vari(i), including geni, osservatori, omicidi, secchii... – DaG Nov 20 '13 at 23:53
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    @DaG: secchii e varii??? Seriously? I would definitely take those as spelling errors. I have seen assassini e assassinii used to distinguish between the plurals of assassino and assassinio, but still it is quite rare. – nico Nov 21 '13 at 8:11
  • I have never seen secchii being used as plural of secchio, since secchi is an adjective. There is no possibility of confusion between rami secchi and i secchi riempiti di acqua e detersivo. – kiamlaluno Nov 21 '13 at 9:11
  • “There is no possibility of confusion” is not in itself alone always a reason to justify orthographic norms. For instance, da (from) and (he gives) can hardly be confused, but they are written differently. And La vecchia porta la sbarra teaches that ambiguity is always behind the corner. – DaG Nov 21 '13 at 9:33
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    @DaG: no possibility of confusion may not be a criterion, but then again possibility of confusion does not justify putting a double i either... – nico Nov 21 '13 at 9:53

I'd add that if an adjective follows, like "oli vegetali", the form with a single "i" is commoner (curiosly, a quick googling shows that "olii vegetali" is mainly used by people which write about organic products and the like)

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    Exactly. Some articles about organic food with "olii vegetali" were actually the reason, why I've asked about it in the first place. – I.M. Nov 21 '13 at 9:15
  • Well, I don't know about everyone else, but personally, whenever I hear this, I tend to hear both of the "ii" in "oli". Most of the time I hear "olii". There may be variations in the way peole pronounce this word, and these may or may not be regional variations. – John Sonderson Jan 21 '15 at 2:29

One should always check and double-check what Wikipedia says, but in this particular case, I'd agree with its distinction between compulsory -ii in the case of a stressed i in -io, and multiple possibilities (-i, -ii, and ) for an unstressed one.

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    Because it's horrible and has nothing to do with Italian orthography. It's an accent borrowed at some point in time from French. You missed another used orthography, though: olj, which Pirandello would probably have used. – egreg Nov 20 '13 at 18:20
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    @egreg To me, olj somehow looks even worse that olî... – I.M. Nov 20 '13 at 18:31
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    I never heard “horrible” and “circumflex accent” in the same argument. Every day teaches us something! :-) – DaG Nov 20 '13 at 23:50
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    @MatteoItalia Is it possible that Pirandello and Verga's usage of j was related to some Sicilian language peculiarity? – martina Nov 21 '13 at 10:04
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    @egreg Are you sure the circumflex accent comes from French? However, it's pretty standard even if very formal, and Italian has imported stuff from everyhwere... And no, I don't believe that the usage of j has much to do with Sicily. The j was a standard way to indicate the semi-consonantic i like in buio / bujo and an unstressed ii can be seen as ji (* buji > buj). – Mauro Vanetti Nov 21 '13 at 10:55

In Italy we never use olî. Also olii is not so common, even if it's better than oli.

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    that's not really an answer. – I.M. Nov 21 '13 at 8:36
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    the question was «Any rule of thumb I could use?» The answer is «Don't use olî, and prefer oli to olii». For me it is a bona fide answer. – mau Nov 21 '13 at 10:49
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    @mau Here at SE network, I'd call an answer only those explanations, which (a) provide some reliable references; (b) explain why one form is preferred to another. "We never" and "not so common but better" don't make a good answer. – I.M. Nov 21 '13 at 11:14
  • I agree that oli is better than olii, but only stating this doesn't make this an answer. – egreg Nov 21 '13 at 22:32
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    @Napolux The thing is: everybody here, including myself, speaks Italian every day, so we all are perfectly aware of what is "common", "normal" and "usual". Both the rules of the SE network and normal logic require that every answer here is supported with some references and explanations. Besides, you statement "we never" was plainly wrong: it's old orthography, which is no longer common but still can be seen in some books and even dictionaries. – I.M. Nov 22 '13 at 22:16

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