Plural form of words ending in -us

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In English, the plural form of words ending in -us, especially those derived from Latin, replaces -us with -i. There are many exceptions, some because the word does not derive from Latin, and others due to custom (e.g., campus, plural campuses). Conversely, some non-Latin words ending in -us and Latin words that did not have their Latin plurals with -i form their English plurals with -i. Some words' plurals end in -i even though they are not Latin, or that is not the Latin plural, e.g., octopi as a plural for octopus. Prescriptivists consider these forms incorrect, but descriptivists may simply describe them as a natural evolution of language.


In Latin, most second declension masculine nouns in -us form their plural in -i. However, some Latin nouns ending in -us are not second declension (cf. Latin grammar). For example, third declension neuter nouns such as opus and corpus have plurals opera and corpora, and fourth declension masculine and feminine nouns such as sinus and tribus have plurals sinūs and tribūs.

Some English words derive from Latin idiosyncratically. For example, bus is a shortened form of omnibus 'for everyone', the dative plural of omnis, and ignoramus is a verb form, 'we do not know'.




Nonetheless, the question of what the Latin plural of virus would have been in ancient times turns out not to be straightforward, as no plural form is attested in ancient Latin literature. Furthermore, its unusual status as a neuter noun ending in -us and not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.[citation needed]

Mass noun in Latin[edit]

The Latin word vīrus (the ī indicates a long i) means "1. slimy liquid, slime; 2. poison, venom", denoting the venom of a snake. This Latin word is probably related to the Greek ἰός (ios) meaning "venom" or "rust" and the Sanskrit word viṣam meaning "toxic, poison".[3]

Since vīrus in antiquity denoted something uncountable, it was a mass noun. Mass nouns pluralize only under special circumstances, hence the non-existence of plural forms in the texts.[4]

There is no known plural for this word in Classical Latin. It is unclear how a plural might have been formed under Latin grammar in ancient times if the word had acquired a meaning requiring a plural form. In Latin, vīrus is generally regarded as a neuter of the second declension, but neuter second declension nouns ending in -us (rather than -um) are rare enough that inferring rules is difficult. (One of the rare attested plurals, pelage as a plural of pelagus, is borrowed from Greek, so does not give guidance for virus.) Plural neuter nouns of other declensions always end in -a (in the nominative, accusative and vocative).

In Neo-Latin, a plural form is necessary in order to express the modern concept of ‘viruses’, which lead to the following declension:[5][6][7]

singular plural
vīrus vīra
genitive vīrī
(antique, heteroclitic: vīrus)
vīrō vīrīs

Treating vīrus as 2nd declension masculine[edit]



The form vīriī is impossible as a plural of vīrus since we only find the ending -iī in the plural form of masculine and feminine words ending in -ius. For instance, radius is pluralized by removing -us, to isolate the stem radi-, and then adding the plural suffix . Thus the -iī ending of the resulting word radiī is not a suffix: it is simply the consequence of adding the actual suffix ī to a stem that has an i as its last letter. Vīriī would be the plural form of the putative, nonexistent word vīrius.

Latin nouns in English[edit]

Some English words of Latin origin do not commonly take the Latin plural, but rather the regular English plurals in -(e)s: campus, bonus, and anus; while others regularly use the Latin forms: radius (radii) and alumnus (alumni). Still others may use either: corpus (corpora or corpuses), formula (formulae in technical contexts, formulas otherwise), index (indices mostly in technical contexts, indexes otherwise).

The form viruses appears in the official Scrabble words list,[8] but neither viri nor virii does. Similarly, the spellchecker built into the Mozilla Firefox browser[9] accepts viruses but neither viri nor virii.

Use of the form virii[edit]





There are three plural forms of octopus: octopuses (/ˈɒktəpʊsɪz/), octopi (/ˈɒktəpaɪ/), and octopodes (/ˈɒktəpəʊdz/, /ˌɒkˈtɒpədiːz/, or /ˌɒktəˈpəʊdeɪz/). Currently, octopuses is the most common form in the UK as well as the US; octopodes is rare, and octopi is often objected to as incorrect.[11]

The Oxford English Dictionary[12] lists octopuses, octopi and octopodes (in that order); it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun, which it is not. Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους), gender masculine, whose plural is oktṓpodes (ὀκτώποδες). If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedēs, analogous to centipedes and mīllipedēs, as the plural form of pēs ('foot') is pedēs. In modern Greek, it is called khtapódi (χταπόδι), gender neuter, with plural form khtapódia (χταπόδια).

Chambers 21st Century Dictionary[13] and the Compact Oxford Dictionary[14] list only octopuses, although the latter notes that octopodes is "still occasionally used"; the British National Corpus has 29 instances of octopuses, 11 of octopi and 4 of octopodes. Merriam-Webster 11th Collegiate Dictionary lists octopuses and octopi, in that order; Webster's New World College Dictionary lists octopuses, octopi and octopodes (in that order).




The situation with the word platypus is similar to that of octopus; the word is etymologically Greek despite its Latinized ending, and so pluralizing it as if it were Latin (i.e. as platypi) is ill-considered. As with octopus, importing Greek morphology into English would have platypodes as the plural, but in practice this form is hardly attested outside of discussions about pluralization. In scientific contexts, biologists often use platypus as both the singular and plural form of the word, in the tradition of sheep or fish, but laypersons and scientists alike often use the simple English plural platypuses. Different dictionaries making different recommendations.

Botanical Latin[edit]

As a word in Botanical Latin (as distinct from Classical Latin), cactus follows standard Latin rules for pluralization and becomes cacti, which has become the prevalent usage in English. Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural.[16] Cactuses is also an acceptable plural in English.

Facetious formations[edit]

See also: Dog Latin



  3. ^ Chambers's Etymological Dictionary Enlarged Edition 1931
  5. ^ Nuntii Latini: Finnish Broadcasting Company (Radiophonia Finnica Generalis). Archiv I. 19.5.2000 - 6.12.2002: "NOVUM VIRUS COMPUTATORIUM
    Novum viri computatorii genus nomine Code Red in praesenti in Interreti grassatur, ut nuntiavit institutum SANS, cuius est securitati retis informatici providere. Code Red II, quod per cursum electronicum diffunditur, priore viro acerbius est et, postquam in servitoria penetravit, in systema lacunam facit. Ita fieri potest, ut alia vira eaque etiam periculosiora in machinas computatorias irrepant. Iam vermis Code Red I molestissimus fuit, cum biduo in trecenta milia computatrorum in omni orbe terrarum invasit."
  6. ^ Pons: virus
  8. ^ OSW Official Scrabble Words (1989) Chambers
  13. ^ Chambers 21st Century Dictionary. Retrieved October 19, 2007.
  14. ^ Compact Oxford Dictionary Retrieved October 19, 2007.
  15. ^ Burchfield, R.W., Fowler's Modern English Usage, Oxford University Press, 539.
  16. ^ Random House Unabridged Dictionary (2006)
  18. ^ "GREEN: Toyota Readies Fleet Of Priuses", John Voelcker