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.^[3] 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.

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:

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")

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 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.

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 / ambi (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").^[9]

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.^[10]

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).^[11] 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.

Germanic languages

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.^[12] 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).^[13]

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 living 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.^[14]

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:

1. 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.
2. 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:

1. With the numeral "one", both the noun, adjective, and numeral were in the same singular case, with the numeral being declined as an pronoun.
2. 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.
3. With the numerals "three" and "four", the noun, adjective, and numeral were in the same plural case.
4. 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:

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:

1. 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.^[16]
2. 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 nominative plural (in the case of Belarusian) or genitive singular (in the case of Serbo-Croatian).^[17] Higher numerals are followed by a noun in the genitive plural.^[18]
3. 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.^[19] 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.
4. 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".^[20]

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.