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Is the phrase (as used in English) "modus operandi" singular or plural? And if the former, what is its plural form [or vice versa]?

(To my untutored eye, "modus" seems to be a singular form, while "operandi" seems to be a plural.)

A search of this site does not reveal anything bearing on the matter.

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    Had you done sufficient but simple research, you would not have needed to ask the question, Plural = modi operandorum: en.wikipedia.org/wiki/Modus_operandi
    – Blessed Geek
    Commented Mar 29, 2015 at 22:20
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    @BlessedGeek: I have to keep spamming people here, but operandorum is wrong. The plural is modi operandi: operandi is just a word in the genitive that doesn't change. "One mode of operation, two modes of operation" (not "two modes of operations"). We don't say a way of life, different ways of lives, do we? Besides, in Latin, you absolutely cannot pluralise a gerundium, and operandi is a gerundium. You can pluralise a gerundivum, but that is very uncommon in the genitive; and this isn't a gerundivum, even though gerundium and neuter gerundivum have the same form.
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 3:11
  • All of the answers seem basically to give the Latin plural. But the phrase is now an English phrase. I'd be interested to read an answer giving what the most natural English plural form would be.
    – curiousdannii
    Commented Mar 30, 2015 at 3:20
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    @BlessedGeek I'm not sure why you would give a reference that contradicts rather than supports your claim. Perhaps you are withholding evidence?
    – Canis Lupus
    Commented Mar 30, 2015 at 4:13
  • @CanisLupus: I edited the Wikipaedia article after he had posted that.
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 18:13

4 Answers 4

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Modus operandi is singular in both Latin and English. The plural is modi operandi, and, judging from this Ngram, I would advise against modus operandis.

Since there seems to be some confusion over why only modus changes form in the plural, but never operandi, I'll explain that too.

Modus is the most important word here, quite clearly meaning mode. It's a noun and nouns are simple. The plural is modi.

Operandi is rather more complex. We may translate it as of operation, but that's not very precise: mode of operation would be modus operationis. More exactly, operandi means of operating. It's a gerund, which can't be pluralized either in Latin or in English. However many times you've gone fishing, you'll still say that you like fishing, never that you like fishings. It's the same in Latin.

The confusion arises because the Latin gerund shares its singular forms with the gerundive, which can be pluralized. We know that modus operandi does not use the gerundive because then it would mean mode of something to be performed, which is rather nonsensical. So, I'd strongly advise against using modi operandorum, since I presume you don't mean modes of things to be performed - nor am I particularly sure what anyone might mean by it.

You might say that, if people used modus operandorum often enough, then using it would be fair game. You'd be right. Luckily for us, however, Ngrams doesn't have a single instance of it.

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    The Ngram search engine isn't perfect. Here are books listed by Google that include modi operandorum.
    – Mike
    Commented Mar 30, 2015 at 7:33
  • I wonder if perhaps a discussion about what type of act should be done next in the Vatican operatic society might meaningfully talk about a modus operandorum ‘form of things to be opera’ed’…
    – Janus Bahs Jacquet
    Commented Mar 30, 2015 at 7:42
  • @WS2 I'm sorry for Ninja-editing Wikipaedia on you. I didn't mean it the way it looks, but I just had to correct the article when I saw it.
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 17:16
  • @Mike Playing devil's advocate... The phrase is originally Latin, but has been co-opted into English. So, if everyone started using "modi operandorum", it'd become correct English by definition. In the same way that so much of English is already terrible Latin, this would be a drop in the proverbial bucket. The N-gram viewer sees all...
    – Parthian Shot
    Commented Mar 30, 2015 at 19:52
  • @ParthianShot: Hello, fellow underworldling. That would be "correct" using a definition not many people would want to write by. In other words, if "correct" just means "what many people do", then it has lost all meaning. Then say "many people do this" rather than applying a judgement of value like "correct". In my inferior (get it?) opinion, most people who ask for writing advice want to write in a nice way, abiding by more rules than the absolute minimum.
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 22:58
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It is singular. The plural form is modi operandi (modes of operation). See where it says plural in Merriam-Webster's, Dictionary.com's, and Wikipedia's entries.

Of note, modi operandorum (modes of operations) is never correct, as plural gerundiums are an impossibility in Latin[1][2].

Sources:

[1] See comments to this answer made by Cerberus, a scholar in the field.

[2] See this page on gerunds in Latin.

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    Operandorum is wrong. The plural is modi operandi: operandi is just a word in the genitive that doesn't change upon pluralisation. "One mode of operation, two modes of operation" (not "two modes of operations"). We don't say a way of life, different ways of lives, do we?
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 3:07
  • @Cerberus, of course there can be "multiple modes of operations", if referring to multiple operations, each of which have multiple modes.
    – Mike
    Commented Mar 30, 2015 at 3:12
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    Sure, that is possible; but modes of operations is not the plural of mode of operation, but it is the plural of mode of operations, which is something else (and something rather odd to boot).
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 3:13
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    @Mike: I have studied Latin in school and at university and as a job for two decades, so I know how it works. Your latest edit has improved your answer, but *modi operandorum is still impossible in Latin. // I see what you mean, but I still don't think modes of operations is the plural of mode of operation, but of mode of operations. One bag of tea, two bags of tea. One bag of teas means something else (now there are suddenly several kinds of tea inside the bag, or something), and its plural two bags of teas does, too.
    – Cerberus - Reinstate Monica
    Commented Mar 30, 2015 at 3:22
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    The biggest problem here is that operandi is a gerund, which can't even be pluralized. The tricky part is that the gerund shares many forms with the gerundive, which can be. Modi operandorum means methods of things to be performed.
    – Anonym
    Commented Mar 30, 2015 at 4:20
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It is a singular phrase. The plural form being "modi operandi".

You can find more about it at Merriam-Webster and TFD

  • modus operandi - "A method of operating or functioning, a person's manner of working. TFD
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Modus operandi = the method of operating. (Plural: modi operandi , but NOT modi operandorum which is the genitive plural of the gerundive, not of the gerund. As others have said, you cannot pluralize a gerund in Latin. Otherwise in this case it would be of operatings, which is a nonsense.)

However: using the Latin noun for operation (operatio, operationis), as far as I know, you could have mode of operation, modes of operation, or mode of operations or modes of operations; each of the four being a slightly different concept. This could be achieved using the genitive of the noun operatio: that is genitive singular = operationis; and genitive plural operationem. Thus the four different possibilities would be modus operationis (mode of operation), modi operationis (modes of operation), modus operationem (mode of operations) and modi operationem (modes of operations). Does that make sense? But wouldn't it be far easier just to say it in English?

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