In linguistics, number is a grammatical category that specifies the quantity of a noun or affects the form of a verb or other part of speech depending on the quantity of the noun to which it refers. Grammatical number is distinct from the use of numerals to specify the exact quantify of a noun; number is usually vague. The most common scheme is singular (one thing) contrasted with plural (many things). Other possibilities are dual number, expressing the existence of precisely two instances of the noun, trial number for three of a noun, paucal number for few but not of a noun, or a collective number that expresses the whole class of the nouns (e.g., mankind).
Languages that distinguish grammatical number commonly do so by inflection. Verbs and other parts of speech may be inflected to agree with the noun. English does this in a limited way: "he sleeps" but "I sleep"; "this chair" but "these chairs". Even in languages such as Mandarin Chinese that do not mark common nouns for grammatical number, pronouns usually have distinct singular and plural forms. Arguably this is not quite the same concept as grammatical number, since we is not the same as multiple instances of I.
English is typical of languages that have singular and plural number. An English plural can correspond to a dual, trial, paucal, or plural in other languages. Here are some irregular examples of singular-plural pairs:
foot (singular), feet (plural)
mouse (singular), mice (plural)
I (singular), we (plural)
And one regular example:
encyclopedia (singular), encyclopedias (plural)
Non-borrowed English irregular nouns come in several forms:
Some voice a final fricative when in plural:
knife, knives (f>v)
mouth, mouths (T>D)
house, houses, (unique plural, s>z)
These plurals are distinct in pronunciation from the possessive. There is also a trend in some areas to regularize some of these nouns.
Survivors of the Old English weak masculine declination add -en:
auroch, aurochen (archaic)
Other -en adders are irregular for different reasons:
eye, eyen (rare)
cow, kine (rare)
brother, brethren (or brothers)
Some nouns have no plural, or are identical when plural and singular:
fish (or fishes)
Pronouns are irregular precisely because they are so common:
he she it, they
Some nouns are rather transparently irregular because they undergo the process of umlaut:
There are several different kinds depending in the starting and ending vowel, but generally, they converge on /i/.
In terms of pronunciation, however, the majority of nouns (and adjectives) in French are not actually declined for number. The -s suffix is not actually pronounced unless the next word starts with a vowel (this is called liaison) and thus does not really show anything; the plural article or other word is the real indicator of plurality. However, plurals still exist in French because irregular nouns, such as those that end in -l such as cheval (horse) form plurals in a different way. Cheval is pronounced [S@val], chevaux is pronounced [S@vo], and this really shows number differences. The same is true for adjectives.
Not only nouns can be declined by number. In many languages, adjectives are declined according to the number of the noun they modify. For example, in French, one may say un arbre vert (a green tree), and des arbres verts ([some] green trees). The word vert (green), in the singular, becomes verts for the plural (unlike English green, which remains green).
In many languages, verbs are conjugated by number as well. Using French as an example again, one says je vois (I see), but nous voyons (we see). The verb voir (to see) in the first person changes from vois in singular, to voyons in plural. In English this occurs in the third person (she runs, they run) but not first or second.
Normally verbs agree with their subject noun in number. But in Ancient Greek and Sanskrit neuter plurals took a singular verb. In English, or at least British English, nouns collectively referring to people may take singular verbs, as the committee are meeting; use of this varies by dialect and level of formality.
Other qualifiers may also agree in number. The English article the does not, the demonstratives this, that do, becoming these, those, and the article a, an is omitted or changed to some in the plural. In French and German, the definite articles have gender distinctions in the singular but not the plural. In Portuguese, the indefinite articleum, uma has plural forms uns, umas.