Dual (abbreviated DU) is a grammatical number that some languages use in addition to singular and plural. When a noun or pronoun appears in dual form, it is interpreted as referring to precisely two of the entities (objects or persons) identified by the noun or pronoun acting as a single unit or in unison. Verbs can also have dual agreement forms in these languages.
The dual number existed in Proto-Indo-European, persisted in many of its descendants, such as Ancient Greek and Sanskrit, which have dual forms across nouns, verbs, and adjectives, Gothic, which used dual forms in pronouns and verbs, and Old English (Anglo-Saxon), which used dual forms in pronouns. It can still be found in a few modern Indo-European languages such as Irish, Scottish Gaelic, Lithuanian, Slovene, and Sorbian languages.
The majority of modern Indo-European languages, including modern English, however, have lost dual through their development and only show residual traces of it. In all these languages, its function has mostly been replaced by simple plurals, although the remnants are evident in the English distinctions: both vs. all, either vs. any, neither vs. none, and so on. A commonly used sentence to exemplify dual in English is "Both go to the same school." where both refers to two specific people who had already been determined in the conversation.
Many Semitic languages also have dual numbers. For instance, in Hebrew יים- (-ayim) or a variation of it is added to the end of some nouns, e.g. some parts of the body (eye, ear, nostril, lip, hand, leg) and some time periods (minute, hour, day, week, month, year) to indicate that it is dual (regardless of how the plural is formed). A similar situation exists in classical Arabic, where ان -ān is added to the end of any noun to indicate that it is dual (regardless of how the plural is formed).
Many languages make a distinction between singular and plural: English, for example, distinguishes between man and men, or house and houses. In some languages, in addition to such singular and plural forms, there is also a dual form, which is used when exactly two people or things are meant. In many languages with dual forms, the use of the dual is mandatory as in some Arabic dialects using dual in nouns as in Hejazi Arabic, and the plural is used only for groups greater than two. However, the use of the dual is optional in some languages such as other modern Arabic dialects including Egyptian Arabic.
In other languages such as Hebrew, the dual exists only for words naming time spans (day, week, etc.), a few measure words, and for words that naturally come in pairs and are not used in the plural except in rhetoric: eyes, ears, and so forth.
In Slovene, the use of the dual is mandatory except for nouns that are natural pairs, such as trousers, eyes, ears, lips, hands, arms, legs, feet, kidneys, breasts, lungs, etc., for which the plural form has to be used unless one wants to stress that something is true for both one and the other part. For example, one says "oči me bolijo" (my eyes hurt), but if they want to stress that both their eyes hurt, they say "obe očesi me bolita". When using the pronoun "obe/oba" (both), the dual form that follows is mandatory.
Although relatively few languages have the dual number, using different words for groups of two and groups greater than two is not uncommon. English has words distinguishing dual vs. plural number, including: both/all, either/any, neither/none, between/among, former/first, and latter/last. Japanese, which has no grammatical number, also has words dochira (which of the two) and dore (which of the three or more), etc.
Use in modern languages
Among living languages, Modern Standard Arabic has a mandatory dual number, marked on nouns, verbs, adjectives and pronouns. (First-person dual forms, however, do not exist; compare this to the lack of third-person dual forms in the old Germanic languages.) Many of the spoken Arabic dialects have a dual marking for nouns (only), and its use can be mandatory in some dialects, and not mandatory in others. Likewise, Akkadian had a dual number, though its use was confined to standard phrases like "two hands", "two eyes", and "two arms". The dual in Hebrew has also atrophied, generally being used for only time, number, and natural pairs (like body parts) even in its most ancient form.
Austronesian languages, particularly Polynesian languages such as Hawaiian, Niuean and Tongan, possess a dual number for pronouns but not for nouns, as nouns are generally marked for plural syntactically and not morphologically. Other Austronesian languages, particularly those spoken in the Philippines, have a dual first-person pronoun; these languages include Ilokano (data), Tausug (kita), and Kapampangan (ìkatá). These forms mean "we", but specifically "you and I". This form once existed in Tagalog (katá or sometimes kitá) but has disappeared from standard usage (save for certain dialects such as in Batangas) since the middle of the 20th century, with kitá as the only surviving form (e.g. Mahál kitá, loosely "I love you").
The dual was a standard feature of the Proto-Uralic language, and lives on in the Samoyedic languages and in most Sami languages, while other branches like Finnish, Estonian and Hungarian have lost it. Sami also features dual pronouns, expressing the concept of "we two here" as contrasted to "we". Nenets, two closely related Samoyedic languages, features a complete set of dual possessive suffixes for two systems, the number of possessors and the number of possessed objects (for example, "two houses of us two" expressed in one word).
The dual form is also used in several modern Indo-European languages, such as Irish, Scottish Gaelic, Slovene and Sorbian (see below for details). The dual was a common feature of all early Slavic languages around the year 1000.
In Modern Standard Arabic, as well as in Classical Arabic, the use of dual is compulsory when describing two units. For this purpose, ان -ān, is added to the end of any noun or adjective regardless of gender or of how the plural is being formed. In the case of feminine nouns ending with ة ta marbuta, this letter becomes a ت ta. When the dual noun or adjective is rendered in the genitive or accusative forms, the ان -ān becomes ين -ain.
Besides the noun and adjective dual, there are also dual verb forms of compulsory use for second and third person, together with their pronouns, but none for the first person.
The use of dual in spoken Arabic varies widely and is mostly rendered a ين -ain even when in nominative context. Whereas its use is quite common in Levantine Arabic, for instance كيلوين kilowain meaning "two kilograms", dual forms are generally not used in Maghrebi Arabic, where two units are commonly expressed with the word زوج zuʒ, as in زوج كيلو zuʒ kilu meaning "a pair of kilograms", with the noun appearing in singular.
Biblical and Mishnaic Hebrew
In Biblical, Mishnaic, and Medieval Hebrew, like Arabic and other Semitic languages, all nouns can have singular, plural or dual forms, and there is still a debate whether there are vestiges of dual verbal forms and pronouns. However, in practice, most nouns use only singular and plural forms. Usually ־ים -īm is added to masculine words to make them plural for example ספר / ספרים sēfer / səfārīm "book / books", whilst with feminine nouns the ־ה -ā is replaced with ־ות -ōṯ. For example, פרה / פרות pārā / pārōṯ "cow / cows". The masculine dual form is shown in pointed text with a pathach; in a purely consonantal text, masculine dual is not indicated at all by the consonants. The dual for (two) days is יוֹמַ֫יִם with pathach under the mem. An example of the dual form is יום / יומיים / ימים yōm / yomạyim / yāmīm "day / two days / [two or more] days". Some words occur so often in pairs that the form with the dual suffix -ạyim is used in practice for the general plural, such as עין / עינים ʿạyin / ʿēnạyim "eye / eyes", used even in a sentence like "The spider has eight eyes." Thus words like ʿēnạyim only appear to be dual, but are in fact what is called "pseudo-dual", which is a way of making a plural. Sometimes, words can change meaning depending on whether the dual or plural form is used, for example; ʿayin can mean eye or water spring in the singular, but in the plural eyes will take the dual form of ʿenayim whilst springs are ʿeynot. Adjectives, verbs, and pronouns have only singular and plural, with the plural forms of these being used with dual nouns.
In Modern Hebrew as used in Israel, there is also a dual number, but its use is very restricted. The dual form is usually used in expressions of time and number. These nouns have plurals as well, which are used for numbers higher than two, for example:
|פעם páʿam ("time", frequency)||פעמיים paʿamáyim ("twice")||שלוש פעמים shalosh pəʿamim ("thrice")|
|יום yom ("day")||יומיים yomáyim ("two days")||שלושה ימים shəlosha yamim ("three days")|
|שנה shaná ("year")||שנתיים shnatáyim ("two years")||שלוש שנים shalosh shanim ("three years")|
|שבוע shavúaʿ ("week")||שבועיים shəvuʿáyim ("two weeks")||שלושה שבועות shəlosha shavuʿot ("three weeks")|
|מאה meʾa ("one hundred")||מאתיים matáyim ("two hundred")||שלוש מאות shalosh meʾot ("three hundred")|
The pseudo-dual is used to form the plural of some body parts, garments, etc., for instance:
- רגל régel ("leg") → רגליים ragláyim ("legs")
- אוזן ózen ("ear") → אוזניים oznáyim ("ears")
- שן shen ("tooth") → שניים shináyim ("teeth")
- מעי məʿi ("intestine") → מעיים məʿáyim ("intestines")
- נעל náʿal ("shoe") → נעליים naʿaláyim ("shoes")
- גרב gérev ("sock") → גרביים garbáyim ("socks")
In this case, even if there are more than two, the dual is still used, for instance יש לכלב ארבע רגליים yesh lə-ḵélev arbaʿ ragláyim ("a dog has four legs").
Another case of the pseudo-dual is duale tantum (a kind of plurale tantum) nouns:
- נקודתיים nəkudatáyim ("colon", lit. "two dots")
- אופניים ofanáyim ("bicycle", lit. "two wheels")
- משקפיים mishkafáyim ("eyeglasses", lit. "two lenses")
- שמיים shamáyim ("sky")
- מספריים misparáyim ("scissors")
In Indo-European languages
The category of dual can be reconstructed for Proto-Indo-European, the ancestor of all Indo-European languages, and it has been retained as a fully functioning category in the earliest attested daughter languages. The best evidence for the dual among ancient Indo-European languages can be found in Old Indo-Iranian (Vedic Sanskrit and Avestan), Homeric Greek and Old Church Slavonic, where its use was obligatory for all inflected categories including verbs, nouns, adjectives, pronouns and some numerals. Various traces of dual can also be found in Gothic and Old Irish (see below), and in some fossilized terms in Latin.
Due to the scarcity of evidence, the reconstruction of dual endings for Proto-Indo-European is difficult, but at least formally according to the comparative method it can be ascertained that no more than three dual endings are reconstructible for nominal inflection. Mallory & Adams (2006) reconstruct the dual endings as:
- Nominative/accusative/vocative: *-h₁(e)
- Genitive/ablative: *-h₁(e) / *-oHs
- Dative: *-me / *-OH
- Locative: *-h₁ow
- Instrumental: *-bʰih₁
The Proto-Indo-European category of dual did not only denote two of something: it could also be used as an associative marker, the so-called elliptical dual. For example, the Vedic deity Mitrá, when appearing in dual form Mitrā́, refers to both Mitra and his companion Varuṇa. Homeric dual Αἴαντε refers to Ajax the Greater and his fighting companion Teucer, and Latin plural Castorēs is used to denote both the semi-god Castor and his twin brother Pollux.
Beside nominal (nouns, adjectives and pronouns), the dual was also present in verbal inflection where the syncretism was much lower.
Of living Indo-European languages, the dual can be found in dialects of Scottish Gaelic, but fully functioning as a paradigmatic category only in Slovene, Sorbian, and the Kajkavian and Chakavian forms of Croatian. Remnants of the dual can be found in many of the remaining daughter languages, where certain forms of the noun are used with the number two (see below for examples).
The dual is widely used in Sanskrit, as noted above. Its use is mandatory when the number of objects is two, and the plural is not permitted in this case, with one exception (see below). It is always indicated by the declensional suffix (and some morphophonemic modifications to the root resulting from addition of the suffix).
For nouns, the dual forms are the same in the following sets of cases, with examples for the masculine noun bāla (boy):
- nominative/accusative: bālau
- instrumental/dative/ablative: bālābhyām
- genitive/locative: bālayoḥ
In Sanskrit adjectives are treated the same as nouns as far as case declensions are concerned. As for pronouns, the same rules apply, except for a few special forms used in some cases.
Verbs have distinct dual forms in the three persons in both the ātmanepada and parasmaipada forms of verbs. For instance, the root pac meaning "to cook", takes the following forms in the dual number of the present tense, called laṭ lakāra:
(Note that in Sanskrit, the order of the persons is reversed.)
The one exception to the rigidness about dual number is in the case of the pronoun asmad (I/we): Sanskrit grammar permits one to use the plural number for asmad even if the actual number of objects denoted is one or two (this is similar to the "royal we"). For example, while ahaṃ bravīmi, āvāṃ brūvaḥ and vayaṃ brūmaḥ are respectively the singular, dual and plural forms of "I say" and "we say", vayaṃ brūmaḥ can be used in the singular and dual sense as well.
The dual can be found in Ancient Greek Homeric texts such as the Iliad and the Odyssey, although its use is only sporadic, owing as much to artistic prerogatives as dictional and metrical requirements within the hexametric meter. There were only two distinct forms of the dual in Ancient Greek.
In classical Greek, the dual was all but lost, except in the Attic dialect of Athens, where it persisted until the fifth century BC. Even in this case, its use depended on the author and certain stock expressions.
In Koine Greek and Modern Greek, the only remnant of the dual is the numeral for "two", δύο, dýo, which has lost its genitive and dative cases (both δυοῖν, dyoīn) and retains its nominative/accusative form. Thus it appears to be undeclined in all cases. Nevertheless, Aristophanes of Byzantium, the foremost authority of his time (early 2nd century BC) on grammar and style, and a staunch defender of "proper" High Attic tradition, admonishes those who write dysí (dative, plural number) rather than the "correct" dyoīn (dative, dual number).
The dual was lost in Latin and its sister Italic languages. However, certain fossilized forms remained, for example, viginti (twenty), but triginta (thirty), the words ambo (both, compare Slavic oba), duo / duae with a dual declension.
Reconstructed Common Celtic nominal and adjectival declensions contain distinct dual forms; pronouns and verbs do not. In Old Irish, nouns and the definite article still have dual forms, but only when accompanied by the numeral *da "two". Traces of the dual remain in Middle Welsh, in nouns denoting pairs of body parts that incorporate the numeral two: e.g. deulin (from glin "knee"), dwyglust (from clust "ear").
In the modern languages, there are still significant remnants of dual number in Irish and Scottish Gaelic in nominal phrases containing the numeral dhá or dà (including the higher numerals 12, 22, etc.). As the following table shows, dhá and dà combines with a singular noun, which is lenited. Masculine nouns take no special inflection, but feminine nouns have a slenderized dual form, which is in fact identical to the dative singular.
|lámh ("a hand" in Irish)||dhá láimh ("two hands")||trí lámh ("three hands")|
|clach ("a stone" in Scottish Gaelic)||dà chloich ("two stones")||trì clachan ("three stones")|
Languages of the Brythonic branch do not have dual number. As mentioned above for Middle Welsh, some nouns can be said to have dual forms, prefixed with a form of the numeral "two" (Breton daou- / div-, Welsh dau- / deu- / dwy-, Cornish dew- / diw-). This process is not fully productive, however, and the prefixed forms are semantically restricted. For example, Breton daouarn (< dorn "hand") can only refer to one person's pair of hands, not any two hands from two different people. Welsh deufis must refer to a period of two consecutive months, whereas dau fis can be any two months (compare “fortnight” in English as opposed to “two weeks” or “14 days”; the first must, but the second and third need not, be a single consecutive period).
The dual was present in all the early Germanic languages, as well as in Proto-Germanic. However, the dual had been entirely lost in nouns by that time, and since verbs agreed with nouns in number, so had the third-person dual form of verbs as a result. The dual therefore remained only in the first- and second-person pronouns and their accompanying verb forms.
Gothic retained this situation more or less unchanged. It had markings for the first and second person for both the verbs and pronouns, for example wit "we two" as compared to weis "we, more than two". Old English, Old Norse and the other old Germanic languages had dual marking only in the personal pronouns, but not in the verbs.
The dual has disappeared as a productive form in all the living languages, with loss of the dual occurring in North Frisian dialects only quite recently. In Austro-Bavarian, the old dual pronouns have replaced the standard plural pronouns, for example, accusative enk, you plural (from Proto-Germanic *inkw, *inkwiz). A similar development in the pronoun system can be seen in Icelandic and Faroese. Another remnant of the dual can be found in the use of the pronoun begge ("both") in the Scandinavian languages of Norwegian and Danish, bägge in Swedish and báðir / báðar / bæði in Faroese and Icelandic. In these languages, in order to state "all + number", the constructions are begge to / báðir tveir / báðar tvær / bæði tvey ("all two") but alle tre / allir tríggir / allar tríggjar / øll trý ("all three"). In German, the expression beide ("both") is equivalent to, though more commonly used than, alle zwei ("all two").
Norwegian Nynorsk also retains the conjunction "korgje" ("one of two") and its inverse "korkje" ("neither of two").
A remnant of a lost dual also survives in the Icelandic and Faroese ordinals first and second, which can be translated two ways: First there is fyrri / fyrri / fyrra and seinni / seinni / seinna, which mean the first and second of two respectively, while fyrsti / fyrsta / fyrsta and annar / önnur / annað mean first and second of more than two. In Icelandic the pronouns annar / önnur / annað ("one") and hinn / hin / hitt ("other") are also used to denote each unit of a set of two in contrast to the pronouns einn / ein / eitt ("one") and annar / önnur / annað ("second"). Therefore in Icelandic "with one hand" translates as með annarri hendi not með einni hendi, and as in English "with the other hand" is með hinni hendinni. An additional element in Icelandic worth mentioning are the interrogative pronouns hvor / hvor / hvort ("who / which / what" of two) and hver / hver / hvert ("who / which / what" of more than two).
Among the Baltic languages, the dual form existed but is now nearly obsolete in standard Lithuanian. The dual form Du litu was still used on two-litas coins issued in 1925, but the plural form (2 litai) is used on later two-litas coins.
|vyras ("a man")||vyru ("two men")||vyrai ("men")|
|pirštas ("finger")||pirštu ("two fingers")||pirštai ("fingers")|
|draugas ("a friend")||draugu ("two friends")||draugai ("friends")|
|mergina ("a girl")||mergini ("two girls")||merginos ("girls")|
|einu ("I go")||einava ("We two go")||einame ("We (more than two) go")|
|eisiu ("I will go")||eisiva ("We two will go")||eisime ("We (more than two) will go")|
Common Slavic had a complete singular-dual-plural number system, although the nominal dual paradigms showed considerable syncretism, just as they did in Proto-Indo-European. Dual was fully operable at the time of Old Church Slavonic manuscript writings, and it has been subsequently lost in most Slavic dialects in the historical period.
Of the living languages, only Slovene, the Chakavian form and certain Kajkavian forms of Croatian, and Sorbian have preserved the dual number as a productive form. In all of the remaining languages, its influence is still found in the declension of nouns of which there are commonly only two: eyes, ears, shoulders, in certain fixed expressions, and the agreement of nouns when used with numbers.
In all the languages, the words "two" and "both" preserve characteristics of the dual declension. The following table shows a selection of forms for the numeral "two":
|Common Slavic||*dъva (masc.)
|Belarusian||два dva (masc./nt.)
дзве dzve (fem.)
|двух dvukh (masc./nt.)
дзвюх dzvyukh (fem.)
|двум dvum (masc./nt.)
дзвюм dzvyum (fem.)
|двума dvuma (masc./nt.)|
дзвюма dzvyuma (fem.)
|Russian||два dva (masc./nt.)
две dve (fem.)
|двух dvukh||двум dvum||двумя dvumya (usual form)|
двемя dvemya (seldom used, dialectal; fem. in some dialects)
|Serbo-Croatian||два / dva (masc./nt.)
две / dvije (fem.)
|двају / dvaju (masc.)
два / dva (nt.)
двеју / dviju (fem.)
|двaма / dvama (masc./nt.)2|
двема / dvjema (fem.)
|Slovak||dva (masc. inanim.)
dvaja / dvoch (masc. anim.)
dve (fem., nt.)
|dvoch||dvom||dvoma / dvomi|
|Ukrainian||два dva (masc./nt.)
дві dvi (fem.)
|двох dvokh||двом dvom||двома dvoma|
- In some Slavic languages, there is a further distinction between animate and inanimate masculine nouns. In Polish, for animate masculine nouns, the possible nominative forms are dwaj, or dwóch.
- Variant form for the masculine/neuter locative and instrumental in Serbo-Croatian: двојим(а) / dvоjim(a).
In Common Slavic, the rules were relatively simple for determining the appropriate case and number form of the noun, when it was used with a numeral. The following rules apply:
- With the numeral "one", both the noun, adjective, and numeral were in the same singular case, with the numeral being declined as an pronoun.
- With the numeral "two", both the noun, adjective, and numeral were in the same dual case. There were separate forms for the masculine and neuter-feminine nouns.
- With the numerals "three" and "four", the noun, adjective, and numeral were in the same plural case.
- With any numeral above "four", the numeral was followed by the noun and adjective in the genitive plural case. The numeral itself was actually a numeral noun that was declined according to its syntactic function.
With the loss of the dual in most of the Slavic languages, the above pattern now is only seen in the forms of the numbers for the tens, hundreds, and rarely thousands. This can be seen by examining the following table:
|Common Slavic||*desętь||*dъva desęti||*trije desęte||*pętь desętъ||*sъto||*dъvě sъtě||*tri sъta||*pętь sъtъ|
|Czech||deset||dvacet||třicet||padesát||sto||dvě stě||tři sta||pět set|
|Upper Sorbian||dźesać||dwaceći||třiceći||pjećdźesat||sto||dwě sćě||tři sta||pjeć stow|
The Common Slavic rules governing the declension of nouns after numerals, which were described above, have been preserved in Slovene. In those Slavic languages that have lost the dual, the system has been simplified and changed in various ways, but many languages have kept traces of the dual in it. In general, Czech, Slovak, Polish and Ukrainian have extended the pattern of "three/four" to "two"; Russian, Belarusian and Serbo-Croatian have, on the contrary, extended the pattern of "two" to "three/four"; and Bulgarian and Macedonian have extended the pattern of "two" to all numerals. The resulting systems are as follows:
- In Czech, Slovak, Polish and Ukrainian, numerals from "two" to "four" are always followed by a noun in the same plural case, but higher numerals (if in the nominative) are followed by a noun in the genitive plural.
- In Belarusian and Serbo-Croatian, numerals from "two" to "four" (if in the nominative) are followed by a noun in a form originating from the Common Slavic nominative dual, which has now completely or almost completely merged with the genitive singular. Higher numerals are followed by a noun in the genitive plural.
- In Russian, the form of noun following the numeral is nominative singular if the numeral ends in "one", genitive singular if the numeral ends in "two" to "four", and genitive plural otherwise. As an exception, the form of noun is also genitive plural if the numeral ends in 11 to 14. Also, some words (for example, many measure words, such as units) have a special "count form" (счётная форма) for use in numerical phrases instead of genitive (for some words mandatory, for others optional), for example, восемь мегабайт, пять килограмм and пять килограммов, три ряда́ and три ря́да, and полтора часа́.
- In Bulgarian and Macedonian, all numerals are followed by a noun in a form originating from the Common Slavic nominative dual, which has now been re-interpreted as a "count form" or "quantitative plural".
These different systems are exemplified in the table below where the word "wolf" is used to form nominative noun phrases with various numerals. The dual and forms originating from it are underlined.
|"wolf"||"wolves"||"two wolves"||"three wolves"||"five wolves"|
|Noun form||nom. sing.||nom. plur.||varies|
|Common Slavic||*vьlkъ||vьlci||dъva vьlka (nom. dual)||tri vьlci (nom. pl.)||pętь vьlkъ (gen. pl.)|
|Slovene||volk||volkovi||dva volka (nom. dual)||trije volkovi (nom. pl.)||pet volkov (gen. pl.)|
|Czech||vlk||vlci||dva/tři vlci (nom. pl.)||pět vlků (gen. pl.)|
|dwa/trzy wilki (nom. pl.)
dwaj/trzej wilcy (nom. pl.)
|pięć wilków (gen. pl.)|
|dva/tri vlky (nom. pl.)
dvaja/traja vlci (nom. pl.)
|päť vlkov (gen. pl.) |
piati vlci (nom. pl.)
|Ukrainian||вовк vovk||вовки́ vovký||два/три во́вки dva/try vóvky (nom. pl.)||п'ять вовків p″yat′ vovkiv (gen. pl.)|
|Belarusian||воўк vowk||ваўкі vawki||два/тры ваўкі dva/try vawki (nom. pl.)||пяць ваўкоў pyats′ vawkow (gen. pl.)|
|Russian||волк volk||волки volki||два/три волкa dva/tri volka (gen. sg.)||пять волков pyat volkov (gen. pl.)|
|Serbo-Croatian||вук / vuk||вукови / vukovi (concrete)
вŷци / vûci (abstract)
|два/три вука / dva/tri vuka (gen. sg.)||пет вукова / pet vukova (gen. pl.)|
|Bulgarian||вълк vǎlk||вълци vǎltsi||два/три/пет вълка dva/tri/pet vălka (count form)|
The dual has also left traces in the declension of nouns describing body parts that humans customarily had two of, for example: eyes, ears, legs, breasts, and hands. Often the plural declension is used to give a figurative meaning. The table below summarizes the key such points.
|Czech||Certain body parts and their modifying adjectives require in the instrumental and genitive plural cases dual forms: se svýma očima (instrumental dual: "with one's own (two) eyes") or u nohou (genitive dual: "at the (two) feet"). Colloquial Czech will often substitute the dual instrumental for the literary plural instrumental case.|
|Polish||Oko ("eye") and ucho ("ear") have plural stems deriving from old dual forms, and alternative instrumental and genitive plural forms with archaic dual endings: gen. pl. oczu/ócz/oczów, uszu/uszów; instr. pl. oczami/oczyma, uszami/uszyma. The declension of ręka ("hand, arm") also contains old dual forms (nom./acc./voc. pl ręce, instr. pl. rękami/rękoma, loc. sg./pl. rękach/ręku). The historically dual forms are usually used to refer a person's two hands (dziecko na ręku "child-in-arms"), while the regularized plural forms are used elsewhere. Other archaic dual forms, including dual verbs, can be encountered in older literature and in dialects: Jak nie chceta, to nie musita "If you don't want to, you don't have to".|
|Slovak||In Slovak, the genitive plural and instrumental plural for the words "eyes" and "ears" has also retained its dual forms: očiam/očí and ušiam/uší.|
|Ukrainian||The words "eyes" and "shoulders" had dual forms in the instrumental plural case: очима ochyma ("eyes") and плечима plechyma ("shoulders"). Furthermore, the nominative plural word вуса vusa, which is the dual of вус vus ("whisker"), refers to the moustache, while the true nominative plural word вуси vusy refers to whiskers.|
|Bulgarian||Some words such as ръка răka "hand" use the originally dual form as a plural (ръце rătse).|
|Russian||In Russian the word колено koleno ("knee", "tribe (Israelites)") has different plurals: колена kolena ("Israelites") is pure plural and колени koleni (body part) is a dual form. Some cases are different as well: коленами kolenami vs. коленями kolenyami (instr.pl.).|
Along with the Sorbian languages, Chakavian Croatian, and the extinct Old Church Slavonic, Slovene uses the dual. Although popular sources claim that Slovene has "preserved full grammatical use of the dual," Standard Slovene (and, to varying degrees, Slovene dialects) show significant reduction of the dual number system when compared with Common Slavic. In general, dual forms have a tendency to be replaced by plural forms. This tendency is stronger in oblique cases than in the nominative/accusative: in standard Slovene, genitive and locative forms have merged with the plural, and in many dialects, pluralization has extended to dative/instrumental forms. Dual inflection is better preserved in masculine forms than in feminine forms. Natural pairs are usually expressed with the plural in Slovene, not with the dual: e.g. roke "hands", ušesa ears. The dual forms of such nouns can be used, in conjunction with the quantifiers dva "two" or oba "both", to emphasize the number: e.g. Imam samo dve roki "I only have two hands". The words for "parents" and "twins" show variation in colloquial Slovene between plural (starši, dvojčki) and dual (starša, dvojčka). Standard Slovene has replaced the nominative dual pronouns of Common Slavic (vě "the two of us", va "the two of you", ja/ji/ji "the two of them" [m./f./n.]) with new synthetic dual forms: midva/midve (literally, "we-two"), vidva/vidve, onadva/onidve/onidve.
Nominative case of noun volk "wolf", with and without numerals:
|nom. sg. (wolf)||nom. dual (2 wolves)||nom. pl. (wolves)|
|wolf||2 wolves||3 (or 4) wolves||5(+) wolves (gen. pl.)|
|Slovene||en volk||dva volka||trije volkovi||pet volkov|
The dual is recognised by many Slovene speakers as one of the most distinctive features of the language and a mark of recognition, and is often mentioned in tourist brochures.
For verbs, the endings in the present tense are given as -va, -ta, -ta. The table below shows a comparison of the conjugation of the verb delati, which means "to do, to make, to work" and belongs to Class IV in the singular, dual, and plural.
In the imperative, the endings are given as -iva for the first-person dual and -ita for the second-person dual. The table below shows the imperative forms for the verb hoditi ("to walk") in the first and second persons of the imperative (the imperative does not exist for first-person singular).
As in Slovenian, the Sorbian language (both dialects Upper and Lower Sorbian) has preserved the dual. For nouns, the following endings are used:
|Masculine||Feminine or neuter|
|Nominative, accusative, vocative||-aj/-ej||-e2/-y/-i|
|Dative, instrumental, locative||-omaj||-omaj|
- The genitive form is based on the plural form of the noun.
- The -e ending causes various softening changes to occur to the preceding constant, for further information see the article on Sorbian.
For example, the declension of sin (masculine) and crow (feminine) in the dual in Upper Sorbian would be given as
|hrěch ("sin")||wróna ("crow")|
|Nominative, accusative, vocative||hrěchaj||wrónje|
|Dative, instrumental, locative||hrěchomaj||wrónomaj|
For verbs, the endings in the present tense are given as -moj, -tej/-taj, -tej/-taj. The table below shows a comparison of the conjugation of the verb pisać, which means "to write" and belongs to Class I in the singular, dual, and plural.
Languages with dual number
- Afroasiatic languages
- Austronesian languages
- Indo-European languages
- Ancient Greek
- Germanic languages (only first and second person pronouns and verb forms)
- Insular Celtic languages:
- Old Church Slavonic
- Old East Slavic
- Sorbian languages:
- Pama–Nyungan languages
- Uralic languages
- Other natural languages
- American Sign Language
- Dogrib (only in the first person)
- Hopi (nouns)
- Khoe languages
- Komo language
- Kunama language
- Lakota (only the personal pronouns, always means "you and I")
- Melanesian Pidgin (several related languages)
- Santali (nouns)
- Xavante language
- Constructed languages
- Vossen, Rainer (2013). The Khoesan Languages (1st ed.). New York: Routledge. p. 58. ISBN 978-0-203-08446-5.
- Gary Rendsburg (July 1982). "Dual Personal Pronouns and Dual Verbs in Hebrew". The Jewish Quarterly Review. New Series. 73 (1): 38–58. doi:10.2307/1454459. JSTOR 1454459. S2CID 165915077.
- Haacke, Wilfrid H.G. (2013). "3.2.1 Namibian Khoekhoe (Nama/Damara)". In Vossen, Rainer (ed.). The Khoesan Languages. Routledge. pp. 141–151. ISBN 978-0-7007-1289-2.
- Güldemann, Tom; Anna-Maria Fehn (2014). Beyond 'Khoisan': Historical Relations in the Kalahari Basin (1st ed.). Amsterdam/Philadelphia: John Benjamins Publishing Company. p. 17. ISBN 978-90-272-6992-8.
- For example: ambō "both", and duo "two", the latter with Iambic shortening.
- Ringe (2006, pp. 42)
- Clackson (2007, p. 101)
- Lewis, Henry; Holger Pedersen (1989). A Concise Comparative Celtic Grammar (3rd ed.). Göttingen: Vandenhoeck & Ruprecht. pp. §§246, 468. ISBN 3-525-26102-0. Thurneysen, Rudolf (1993) . A Grammar of Old Irish. Trans. by D. A. Binchy and Osborn Bergin. Dublin Institute for Advanced Studies. ISBN 1-85500-161-6. Evans, D. Simon (1989) . A Grammar of Middle Welsh. Dublin Institute for Advanced Studies. pp. §§30, 33. ISBN 1-85500-000-8.
- Ó Maolalaigh, Roibeard; Iain MacAonghuis (1997). Scottish Gaelic in Three Months. Hugo's Language Books. ISBN 978-0-85285-234-7.
- Heinecke, Johannes (2002). "Is there a Category of Dual in Breton or Welsh?". Journal of Celtic Linguistics. 7: 85–101.
- Howe, Stephen. The Personal Pronouns in the Germanic Languages. A study of personal pronoun morphology and change in the Germanic languages from the first records to the present day. [Studia Linguistica Germanica, 43]. Berlin: de Gruyter, 1996. (xxii + 390 pp.) pp. 193–195.
- Mayer, Gerald L. (1973) "Common Tendencies in the Syntactic Development of 'Two', 'Three,' and 'Four' in Slavic." The Slavic and East European Journal 17.3:308–314.
- These forms are taken from De Bray, R. G. A. Guide to the Slavonic Languages. London, 1951.
- However, Ukrainian is special in that the form used with "two", "three" and "form" has the stress pattern of the genitive singular and thus of the old dual.
- Browne, Wayles and Theresa Alt (2004) A Handbook of Bosnian, Serbian and Croatian.  P.21
- Kordić, Snježana (2006) [1st pub. 1997]. Serbo-Croatian. Languages of the World/Materials ; 148. Munich & Newcastle: Lincom Europa. p. 32. ISBN 3-89586-161-8. OCLC 37959860. OL 2863538W. . Contents. Summary. [Grammar book].
- Paul V. Cubberley (2002) Russian: a linguistic introduction. p.141
- Friedman, Victor (2001) Macedonian.  P.19
- Swan, Oscar E. (2002). A Grammar of Contemporary Polish. Bloomington, IN: Slavica. pp. 57, 199, 216. ISBN 0-89357-296-9.
- "International Mother Language Day". Statistical Office of the Republic of Slovenia. 19 February 2009. Retrieved 3 February 2011.
- Jakop, Tjaša (2008). The Dual in Slovene Dialects. Bochum: Brockmeyer. ISBN 978-3-8196-0705-9.
- Jakop (2008, pp. 104–105)
- Jakop (2008, pp. 6ff)
- Derganc, Aleksandra. 2006. Some Characteristics of the Dual in Slovenian. Slavistična revija 54 (special issue): 416–434; especially pp. 428–429.
- "Khamti." Indian Institute of Technology Guwahati. N.p., n.d. Web. 14 Oct. 2012. <www.iitg.ernet.in/rcilts/phaseI/languages/khamti.htm>
- Otero, Manuel A. "Dual Number in Ethiopian Komo." Nilo-Saharan: Models and Descriptions. By Angelika Mietzner and Anne Storch. Cologne: Rudiger Koppe Verlag, 2015. 123-34. Print.
- Idris, Nikodimos.1987. The Kunama and their language. Addis Ababa University BA thesis.
- Clackson, James (2007). Indo-European Linguistics: An Introduction. New York: Cambridge University Press.
- Fritz, Matthias. Der Dual im Indogermanischen. Heidelberg: Universitätsverlag Winter, 2011.
- Fontinoy, Charles. Le duel dans les langues sémitiques. Paris: Les Belles Lettres, 1969.
- Mallory, James Patrick; Adams, Douglas Q. (2006). The Oxford Introduction to Proto-Indo-European and the Proto-Indo-European World. New York: Oxford University Press.
- Ringe, Donald (2006). From Proto-Indo-European to Proto-Germanic. New York: Oxford University Press.