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).
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.
Among living languages, Modern Standard Arabic has a mandatory dual number, marked on nouns, verbs, adjectives, and pronouns.
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.
The dual form is also used in several modern Indo-European languages, such as Irish, Scottish Gaelic, Slovene, and Sorbian (see below for details).
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.
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.
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.
[7] 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.
Of living Indo-European languages, the dual can be found in dialects of Scottish Gaelic,[8] but fully functioning as a paradigmatic category only in Slovene, and Sorbian.
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 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.
In classical Greek, the dual was lost, except in the Attic dialect of Athens, where it persisted until the fifth century BC.
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).
However, certain fossilized forms remained, for example, viginti (twenty), but triginta (thirty), the words ambo / ambae (both, compare Slavic oba / obě from earlier *abō / *abāi), duo / duae with a dual declension.
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").
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.
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.
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.
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.
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).
[31] 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.
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.
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.