What is the plural of "penis"?
Dear Straight Dope:
What exactly is the plural of penis? I would say penises, but it seems too simple and obvious. Is it penis, like deer, or maybe peni, like fungi?
If you have more than one, you should be writing to Ripley's and not to the Straight Dope. Heck, Sissy, if you have even one, the first thing you should do is change your name to Buddy.
In this case, your first instinct is a good one. The English-style plural is usually acceptable and often preferred. When you don't know what the Latin plural is and don't have a dictionary handy, you should choose the English-style plural rather than try to guess. (Sometimes even dictionaries will steer you wrong; see below). In your example, penises is a perfectly good plural of penis in English. Many people who deal with penises professionally use the Latin-style plural penes instead. That's fine too, but even among urologists, penises seem to predominate. Seems, rather. "Penises" seems to predominate.
Guessing the plural of a Latin word is one of those things where a little learning is a dangerous thing (but that's still "not one half so bad as a lot of ignorance," to quote Terry Pratchett). Those with entirely too much learning know that Latin nouns are divided into five categories, called declensions. To figure out the plural of a Latin noun without cheating (i.e., looking it up), it is necessary, and often sufficient, to know which of the five declensions it belongs to. (There are a few nouns, like virus, that don't fit neatly into any of the declensions, but more on that later). For example, you mention peni as a possible plural of penis. The -i ending is valid for forming the plural of second-declension Latin nouns ending in -us, but of course that doesn't apply to penis. Part of the problem is that when unaccented, the singular endings -us and -is tend to be pronounced the same in English. Those with a little learning know that penus, if it were a second declension noun like most -us nouns in Latin, would be expected to have the plural form peni. Since penus would be pronounced the same--or almost the same--as penis in English, the temptation is strong to use the incorrect peni as the plural. Peni is an example of what is called pseudo-Latin, something that looks like Latin but isn't. A similar mistake is using porpi as the plural of porpoise, but in that case the singular was long spelled porpus under the mistaken impression that it was a Latin word.
Penis is a third declension noun, not second declension. These nouns often end in -is in the singular and -es in the plural. The English style -ises is sometimes preferred. Hence, we have penises (half of us do, anyway), and mantises and pelvises, but only more rarely do you see penes, mantes, and pelves, though they are not incorrect. In many cases, only the Latin form is acceptable: We have testes (some more than others) and crises and psychoses, but never testises, crisises, or pyschosises.
Another group of third-declension Latin nouns, mostly borrowed from Greek, end in -is in the singular, but the full root is not found in that form. These you either have to learn by heart or look up. For example, the Latin plural of iris is irides, which is acceptable in English, but I prefer irises. I like clitorises, but the Latin form clitorides is also acceptable.
Among second declension -us nouns borrowed into English, the English style plural -uses is often preferred to the Latin -i. Thus isthmuses and crocuses are generally preferred, but isthmi and croci are still acceptable. For many words, the Latin style is preferred, but the English is acceptable, as in fungi (or funguses). Sometimes the Latin style is preferred in technical usage, while the English style is better for the nontechnical. Mathematically speaking, ellipses have foci, while investigations may have focuses.
Of course the English style -uses requires an extra syllable, and you have to judge for yourself whether it's worth it for polysyllabic words. Nuclei rolls off the tongue easier than nucleuses, but both are acceptable. Either the long hippopotami or the longer hippopotamuses is acceptable, but it's easy to see why some people are attracted to hippos. In Greek, the African pachyderm was called a riverine horse: hippopotamos (plural hippopotamoi) or hippos potamios (plural hippoi potamioi). Since only the Latinized form in -us is found in English, neither of these Greek plurals is needed. The shortened form hippo is found only in English and can only take the plural hippos. You may sometimes see hippoi used in English as the plural of hippo. Hippoi is properly the Greek plural of hippos ("horse"), not of hippo.
Note that rhinoceros is a pachyderm of a different color. Both words are ultimately from Greek, and the last syllable is pronounced the same in English, but rhinoceri is not proper Latin (nor Greek). That form has found its way into some English dictionaries, but I would advise against messing with rhinoceri. In Latin rhinoceros is a third declension noun with the plural rhinocerotes. Rarely you see the plural form rhinocerontes in English, but that is properly the plural of a variant Latin singular form, rhinoceron. You'll be laughed straight out of the zoo if you try to use either one in English. Stick to rhinoceroses. Or better yet, rhinos (certainly not rhinoi).
One other group of Latin nouns in -us is different. These are fourth-declension masculine nouns. The plural in Latin is spelled the same as the singular, but the u is pronounced long rather than short. When these words are borrowed into English, the English-style plural is almost always preferred over the Latin. We have censuses, hiatuses, sinuses, and fetuses (or foetuses). The second-declension style endings are never correct, so no cenci, hiati, sini, or feti please. Apparatus is rarely pluralized in English. When a plural is needed, either apparatus or apparatuses is acceptable, but never apparati. It so happens that penus, the near homophone of penis that I mentioned above, is a real word in Latin but of the fourth declension, so the plural is penus, not peni. It means "household stock," something you would presumably want to stow along with the family jewels.
Yet another group of Latin nouns in -us follow different rules. If you're still taking notes, these form another subset of the third declension. Typically in these cases the singular does not include the full root. The plural of genus is genera (not genuses and certainly not geni). In English the plural of opus (meaning a creative work) is opera (or opuses). In Latin, opera was originally the plural of opus, but in both Latin and English, opera can correctly be treated as a singular. In English the plural of opera (the thing that ain't over till the fat lady sings) is operas. When opera is used as a singular in Latin (where it meant more of less the same thing as opus), the correct plural is operae, but this is rarely if ever used in English.
Octopus is another of these third declension nouns in Latin, borrowed from Greek. The Latin plural is octopodes, which is acceptable in English, but octopuses (or even octopus) seems more at home in English. The form octopi is quite common in English, but it is pseudo-Latin. It is based on the mistaken belief that octopus was a second-declension noun like fungus. It has made its way into many English dictionaries, but I would not recommend getting tangled up with octopi. It is true that many standard English words have entered the language through mistakes (an apron from a napron, pea from pease, etc.), so octopi may not be totally indefensible. But people who know Latin, admittedly not a large group, will think less of you for using it.
There is one more common English -us word borrowed from Latin that doesn't follow any of the rules above: virus. To the Romans a virus was a dangerous or disgusting substance, anything from snake venom to body odor. Ancient grammarians couldn't agree whether the word was a third-declension noun, a fourth-declension noun or in a class by itself, but the one thing they could agree on was that it didn't have a plural form. Ever. To the Romans, it was a mass noun, not a count noun. That hasn't stopped English writers from inventing pseudo-Latin plural forms to cover the modern countable senses of the word. Viri is formed on the false assumption that virus is a second-declension noun. (Viri in fact is the plural of Latin vir, "man".) Virii is an even worse mistake. Only Latin nouns that end in -ius form the plural with -ii. There are no really common English plurals in -ii other than radii. That hasn't stopped people from trying out such atrocious forms as virii and penii. Virii would be the plural of virius, if such a word existed in Latin. Other suggested plurals include virora, vira, virua, and vire. For more on the debate, see http://www.perl.com/language/misc/virus. html. The one inescapable fact is that in classical Latin, there was no plural of the word. In English, the only correct plural is viruses.
Some English -us nouns were borrowed from other parts of speech in Latin or from other forms (cases) of the noun than the usual nominative. Most of us don't get boni or ride bi (but I'm not judgmental if you do). We get bonuses and ride buses. Bonus is not a noun in Latin but an adjective meaning "good"; bus is a shortened form of omnibus, which is already a Latin dative plural meaning "for all." A few English nouns in -us derive from Latin verbs, so they can't be pluralized like Latin nouns. Mandamus means "we order" in Latin, and ignoramus means "we are ignorant." We can issue mandamuses to ignoramuses, but we can safely ignore mandami from ignorami. And of course not all nouns that end in -us are Latin at all. Walruses may sun themselves on taluses, but you will search tali for walri in vain.
Since we've come this far, we may as well deal with some other common Latin plurals that have found their way into English.
First declension Latin singulars end in -a in the singular and in -ae in the plural. Often the English and Latin style plurals are both acceptable. You have larvae (or larvas) and amoebas (or amoebae). When speaking of female graduates, you say alumnae (but not alumnas). Sometimes which form to use depends on the context. Radios have antennas but insects, unless they're trying to tune in a Rimsky-Korsakov number on the wireless, have antennae. Sometimes the plural form is more common in English than the singular. Minutia is the singular of minutiae and alga is the singular of algae.
There are a few Latin words in -a that do not form the plural in -ae because they are third declension nouns borrowed from Greek. The English style plural is usually best. Traumas, dramas, and dogmas are preferred over traumata, dramata, and dogmata, but stigmata is preferred over stigmas. You should never accept enemata from strangers.
A subset of the second declension ends in -um in the singular and in -a in the plural. In English -ums is sometimes preferred. Forums (or fora), gymnasiums (or gymnasia), podiums (or podia), but bacteria (not bacteriums), phyla (not phylums). Seers are mediums but radio and television are two media. Data is the plural of datum in Latin and English. English also has the plural form datums, but only in the cartographic sense (meaning a reference point). In English, purists still rail against using data and media with a singular verb. These are instances of usages that began as mistakes but are now so common that they are arguably correct. Another example of the same evolution is agenda. In Latin (and sometimes in English) it is the plural of agendum (meaning "a thing that needs to be done") but is now almost invariably treated as a singular in English (meaning "a list or set of things that need to be done"), with the correct English plural agendas. Plural forms that take a further pluralization (correctly or incorrectly) are called "double plurals." Other examples include the incorrect forms alumnis and bacterias and the correct operas. One caution--quorum is a genitive plural pronoun in Latin. The English plural is quorums, never quora.
Latin words ending in -ies are usually the same in the singular and plural, both in Latin and in English. Series, species, caries (as in dental caries) are correct as singular and plural.
Latin singulars ending in -on are borrowings from Greek and usually end in -a in the plural in Latin. Phenomena (or arguably phenomenons, as in "singing phenomenons"), ganglia (or ganglions). Sometimes the English-style plural seems more natural, as in automatons (but automata is acceptable). Criteria is now sometimes used as a singular in English, but it started life as the plural of criterion.
Latin singulars in -x have plurals in -ces or -ges in Latin, but in English -ixes is usually preferred. Dominatrixes (or dominatrices), indexes (or indices), cervixes (or cervices). The bones of the fingers and toes are phalanges but infantry formations are more often phalanxes.
There are several odd words whose plurals will not be obvious from the above rules. One oddball is pubes (two syllables, meaning the pubic region or pubic hair). It is the same in the singular and plural in Latin and English. The supposed singular pubis is pseudo-Latin, except when it means the pubic bone (in full, os pubis, "bone of the pubes"). The plural of specimen in Latin is specimina, but in English specimens. The plural of exemplar is exemplaria in Latin, but exemplars is a better model for English.
Some words from Latin are more common in the plural, so the singular form may not be obvious. Mores (as in folkways and mores) has the rare singular form mos. Viscera (the internal organs) has the rare singular viscus. Insignia and regalia are the plurals of insigne and regale in Latin, but these singulars are rare in English. Paraphernalia is a plural in English and Latin. It has a rare English singular, paraphernal. The Latin singular, not found in English, is paraphernalis. Feces (or faeces) is plural in Latin with the singular fex (or faex). It is usually treated as a singular in English. Stamina started life as the plural of stamen ("thread") but is now usually a singular in English, except sometimes when referring to sexual anatomy (of the botanical kind). Flowers may have many stamina, but Gennifer's former lover has much stamina.
The name of our species, Homo sapiens (literally "wise human") is singular in both Latin and English. The plural of the phrase in Latin--in the non-technical sense of a wise human--would be homines sapientes, but there is never any call to use a plural in English. There is only one species called Homo sapiens. Homo sapien as the supposed singular of Homo sapiens is an abomination. I call this a "double singular." Kudo as the supposed singular of kudos is another example. Borrowed directly from the Greek, kudos ("praise") is already singular. The questionable form congery formed from the Latin singular congeries is another instance. Bicep, tricep, quadricep, and forcep are incorrectly formed from biceps, triceps, quadriceps, and forceps, which are already singular.
Gladiolus has given rise to a double singular too. Gladiolus is a Latin singular with the plural form gladioli. This is probably the best choice for the plural in English, but gladioluses is also acceptable. The similarity of sound of -as and -us in unaccented English syllables makes some people believe the word is gladiolas, which they suppose is a plural with the singular form gladiola. This double singular has made its way into some dictionaries, but the shortened glad would be a happier choice.
Other than Latin words and Greek words mostly borrowed through Latin, few words have been borrowed into English complete with their foreign plurals. In the case of many of the western Romance languages (such as French, Spanish, and Portuguese), it's usually hard to tell because in these languages plurals often end in the letter s, as in English. Linguists believe this form of the plural in western Romance languages is derived from the Latin forms -as, -os, and -es (first, second, and third declension masculine and feminine accusative plurals, used for direct objects) and not, as you might expect, from forms like -es (third declension nominative plurals, used for subjects). The native English pluralizing suffixes spelled -s and -es (but often pronounced with a /z/ sound) come from the same ultimate source (one class of proto-Indo-European plural endings), but by a very different route. It is largely a matter of chance that Old English and Old French happened to retain the same plural endings, almost to the exclusion of the many other plural endings in PIE. After the Norman invasion, the introduction of French -s may have hastened the decline of other plural endings in English, but the process was already underway. We still have oxen and brethren (if we can tell them apart), but we no longer wear shoen and live in housen.
Unlike French and Spanish, Italian gets its plurals from the Latin nominative rather than the accusative. When borrowed into English, these give us such words as graffiti, which is properly the plural of graffito, but which is now often treated as a singular in English. A particularly interesting case is bandit. We anglicized the Italian bandito to bandit in the singular but still sometimes use the Italian plural banditti alongside the English form bandits. Cognoscenti is the plural of the obsolete Italian cognoscente. Also from Italian we get many food terms that are plurals in that language, but treated as singular in English: spaghetti, broccoli, and zucchini.
Some Hebrew plurals such as seraphim and cherubim exist alongside the English-style plurals like seraphs and cherubs. From Arabic, we have jinn (or djinn) as the plural of jinni (or djinni).
Other languages are not often honored by having their plurals accepted on equal terms. For example, you would never say that this Staff Report is longer than many Icelandic sögur; you would say that this Staff Report is longer than many Icelandic sagas. And that, I think, is quite long enough.