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 Its function has mostly been replaced by the simple plural.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 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 an extensive 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 ('my eyes hurt', using plural forms), but if they want to stress that both their eyes hurt, they say (using dual forms). When using the pronoun / ('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 (, 'which of the two') and (, '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 and the lack of first-person dual verb forms in Ancient Greek.) 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 (), Tausug (), and Kapampangan (). These forms mean "we", but specifically "you and I". This form once existed in Tagalog ( or sometimes ) but has disappeared from standard usage (save for certain dialects such as in Batangas) since the middle of the 20th century, with as the only surviving form (e.g. , 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, 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 , this letter becomes a . When the dual noun or adjective is rendered in the genitive or accusative cases, the becomes .
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 even when in nominative context. Whereas its use is quite common in Levantine Arabic, for instance meaning "two kilograms", dual forms are generally not used in Maghrebi Arabic, where two units are commonly expressed with the word , as in 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:
Singular Double Triple ("time", frequency) ("twice") ("thrice") ("day") ("two days") ("three days") ("year") ("two years") ("three years") ("week") ("two weeks") ("three weeks") ("one hundred") ("two hundred") ("three hundred")
The pseudo-dual is used to form the plural of some body parts, garments, etc., for instance:
("leg") → ("legs") ("ear") → ("ears") ("tooth") → ("teeth") ("intestine") → ("intestines") ("shoe") → ("shoes") ("sock") → ("socks")
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:
("colon", lit. "two dots") ("bicycle", lit. "two wheels") ("eyeglasses", lit. "two lenses") ("sky") ("scissors")
Khoe languages In Nama, nouns have three genders and three grammatical numbers.
Singular Dual Plural Feminine piris pirira piridi goat Masculine arib arikha arigu dog Neuter khoe-i khoera khoen people
The non-Khoe Khoesan languages (Tuu and Kxʼa), 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: 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 refers to Ajax the Greater and his fighting companion Teucer, and Latin plural 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.