Dual (abbreviated ) 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 and 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 its 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, have lost the dual number through their development. Its function has mostly been replaced by the simple plural. They may however show residual traces of the dual, for example 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 have dual number. For instance, in Hebrew - 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 is added to the end of any noun to indicate that it is dual (regardless of how the plural is formed). It is also present in Khoisan languages that have a rich inflectional morphology, particularly Khoe languages, as well as Kunama, a Nilo-Saharan language.

Comparative characteristics

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. But the use of "obe (both)" is not mandatory since "očesi (two eyes)" as it is, implies that one means both eyes. 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 (どちら, 'which of the two') and dore (どれ, '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. Inuktitut and the related Central Alaskan Yup'ik language use dual forms; however, the related Greenlandic language does not (though it used to have them). Khoekhoegowab and other Khoe languages mark dual number in their person-gender-number enclitics, though the neuter gender does not have a dual 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 branch and in most Sami languages, while other members of the family like Finnish, Estonian, and Hungarian have lost it. Sami languages also feature 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.

Arabic

In Modern Standard Arabic, as well as in Classical Arabic, the use of dual is compulsory when describing two units. For this purpose, ان is added to the end of any noun or adjective regardless of gender or of how the plural is formed. In the case of feminine nouns ending with ة, this letter becomes a ت. When the dual noun or adjective is rendered in the genitive or accusative cases, the ان becomes ين. 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 as ين even when in nominative context. Whereas its use is quite common in Levantine Arabic, for instance كيلوين meaning "two kilograms", dual forms are generally not used in Maghrebi Arabic, where two units are commonly expressed with the word زوج , as in زوج كيلو meaning "a pair of kilograms", with the noun appearing in singular.

Hebrew

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 is added to masculine words to make them plural for example "book / books", whilst with feminine nouns the is replaced with. For example, "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 with pathach under the mem. An example of the dual form is "day / two days / [two or more] days". Some words occur so often in pairs that the form with the dual suffix is used in practice for the general plural, such as "eye / eyes", used even in a sentence like "The spider has eight eyes." Thus words like 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; can mean eye or water spring in the singular, but in the plural eyes will take the dual form of whilst springs are. Adjectives, verbs, and pronouns have only singular and plural, with the plural forms of these being used with dual nouns.

Modern Hebrew

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: The pseudo-dual is used to form the plural of some body parts, garments, etc., for instance: In this case, even if there are more than two, the dual is still used, for instance יש לכלב ארבע רגליים ("a dog has four legs"). Another case of the pseudo-dual is duale tantum (a kind of plurale tantum) nouns:

Khoe languages

In Nama, nouns have three genders and three grammatical numbers. The non-Khoe Khoesan languages (Tuu and Kx'a), do not have dual number marking of nouns.

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, Old Irish, and Latin (more below). 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. reconstruct the dual endings as: 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, and Sorbian. 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).

Sanskrit

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 (boy): 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: (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.

Greek

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 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 δυσί (dysí) (dative, plural number) rather than the "correct" δυοῖν (dyoīn) (dative, dual number).

Latin

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 / ambae (both, compare Slavic oba), duo / duae with a dual declension.

Celtic languages

Reconstructed Proto-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. 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 modern Welsh term dwylo (= hands) is formed by adding the feminine (and conjoining) form of 'two' (dwy) with the word for 'hand' — llaw becoming lo as it is no longer in a stressed syllable.

Old, Middle, and Modern English

In Proto-Germanic, the dual had been entirely lost in nouns, and since verbs agreed with nouns in number, the third person dual form of verbs was also lost. The dual therefore remained only in the first and second person pronouns and their accompanying verb forms. Old English further lost all remaining dual verbs, keeping only first and second person dual pronouns. The Old English first person dual pronoun was wit in the nominative and unc in the accusative, and the second person equivalents were git and inc respectively. The West Saxon dialect also had the genitive forms of uncer for first person and incer for second person. The dual lasted beyond Old English into the Early Middle English period in the Southern and Midland dialects. Middle English saw git evolve into ȝit, and inc can be seen in various different forms including ȝinc, ȝunc, unk, hunk, and hunke. The dual mostly died out in the early 1200s, surviving to around 1300 only in the East Midland dialect. In a small number of modern English dialects, dual pronouns have independently returned. These include:

Other Germanic languages

Gothic retained the dual more or less unchanged from Proto-Germanic. 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 Norse and other old Germanic languages, like Old English, had dual marking only in the personal pronouns and 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: nominative es, accusative enk (from Proto-Germanic jut and 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 tvö ("all two") but alle tre / allir þrír / allar þrjár / öll þrjú ("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).

Baltic languages

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.

Slavic languages

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 extant Slavic languages, only Slovene, Chakavian and certain Kajkavian dialects, 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": Notes: 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 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: 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: 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. 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.

Slovene

Along with the Sorbian languages, Chakavian, some Kajkavian dialects, 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: 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).

Sorbian language

As in Slovenian, the Sorbian language (both dialects Upper and Lower Sorbian) has preserved the dual. For nouns, the following endings are used: For example, the declension of sin (masculine) and crow (feminine) in the dual in Upper Sorbian would be given as 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