Grammatical number

In linguistics, grammatical number is a feature of nouns, pronouns, adjectives and verb agreement that expresses count distinctions. English and many other languages present number categories of singular or plural. Some languages also have a dual, [|trial] and paucal number or other arrangements.
The word "number" is also used in linguistics to describe the distinction between certain grammatical aspects that indicate the number of times an event occurs, such as the semelfactive aspect, the iterative aspect, etc. For that use of the term, see "Grammatical aspect".

Overview

Most languages of the world have formal means to express differences of number. One widespread distinction, found in English and many other languages, involves a simple two-way contrast between singular and plural number. Discussion of other more elaborate systems of number appears below.
Grammatical number is a morphological category characterized by the expression of quantity through inflection or agreement. As an example, consider the English sentences below:
The quantity of apples is marked on the noun—"apple" singular number vs. "apples" plural number —on the demonstrative, that/those, and on the verb, is/are. In the second sentence, all this information is redundant, since quantity is already indicated by the numeral two.
A language has grammatical number when its noun forms are subdivided into morphological classes according to the quantity they express, such that:

Every noun form belongs to a number class.
Noun modifiers and verbs may also have different forms for each number subclass and inflect to match the number of the nouns they modify or agree with.

This is partly true for English: every noun and pronoun form is singular or plural. Some modifiers of nouns—namely the demonstrative determiners—and finite verbs inflect to agree with the number of the noun forms they modify or have as subject: this car and these cars are correct, while *this cars and *these car are incorrect. However, adjectives do not inflect for number, and many verb forms do not distinguish between singular and plural.
Many languages distinguish between count nouns and mass nouns. Only count nouns can be freely used in the singular and in the plural. Mass nouns, like "milk", "gold", and "furniture", are normally invariant.
Not all languages have number as a grammatical category. In those that do not, quantity must be expressed either directly, with numerals, or indirectly, through optional quantifiers. However, many of these languages compensate for the lack of grammatical number with an extensive system of measure words.
Joseph Greenberg has proposed a number category hierarchy as a linguistic universal: "No language has a trial number unless it has a dual. No language has a dual unless it has a plural." This hierarchy does not account for the paucal.

Geographical distribution

Obligatory plural marking of all nouns is found throughout the languages of western and northern Eurasia and most parts of Africa. The rest of the world's languages present a heterogeneous picture. Optional plural marking is common in Southeast and East Asia and Australian languages, and complete lack of plural marking is particularly found in New Guinea and Australian languages. In addition to the areal correlations, there also seems to be at least one correlation with morphological typology: isolating languages appear to favor no or non-obligatory plural marking. This can be seen particularly in Africa, where optionality or absence of plural marking is found particularly in the isolating languages of West Africa.

Types of number

Singular and plural

One of the simplest number distinctions a language can make is singular and plural. Singular denotes exactly one referent, while plural denotes more than one referent. For example, in English:

dog
dogs

To mark number, English has different singular and plural forms for nouns and verbs : "my dog watches television" and "my dogs watch television". This is not universal: Wambaya marks number on nouns but not verbs, and Onondaga marks number on verbs but not nouns. Latin has different singular and plural forms for nouns, verbs, and adjectives, in contrast to English where adjectives do not change for number. Tundra Nenets can mark singular and plural on nouns, verbs, adjectives, adverbs, and postpositions. The most common part of speech to show a number distinction is pronouns. An example of a personal pronoun system distinguishing singular and plural is that of Wayoró:

	Singular	Plural
First	on	ote
First		txire
Second	en	djat
Third	ndeke	ndeat

Dual

Like the singular denotes exactly one item, the dual number denotes exactly two items. For example, in Camsá:

kes̈"dog"
kes̈at"two dogs"
kes̈ëng"dogs"

In languages with a singular/dual/plural paradigm, the exact meaning of plural depends on whether the dual is obligatory or facultative. In contrast to English and other singular/plural languages where plural means two or more, in languages with an obligatory dual, plural strictly means three or more. This is the case for Sanskrit, North Mansi, and Alutiiq. In languages with a facultative dual, two of something can be referred to using either the dual or the plural, and so plural means two or more. This is the case for modern Arabic dialects, at least some Inuktitut dialects, and Yandruwandha. In some languages, the dual is obligatory in certain cases but facultative in others. In Slovene, it is obligatory for pronouns but facultative for nouns. In Comanche, it is obligatory when referring to humans, facultative for other animate nouns, and rarely used for inanimate nouns.
There are also languages where use of the dual number is more restricted than singular and plural. In the possessive noun forms of Northern Sámi, the possessor can be in the dual number, but the noun possessed can only be singular or plural. Pronouns are the only part of speech with a dual form in some Polynesian languages, including Samoan, Tuvaluan, and Māori. In Maltese, the dual only exists for about 30 specific nouns, of which it is obligatory for only eight. Words that can take a facultative dual in Maltese include egg, branch, tear, and wicker basket. In Mezquital Otomi, the dual can only be used by an adult male speaking to another adult male.

	Singular	Dual	Plural
First	au/ahau	māua	mātou
First		tāua	tātou
Second	koe	kōrua	koutou
Third	ia	rāua	rātou

Dual number existed in all nouns and adjectives of Proto-Indo-European around 4000 BCE, and was inherited in some form in many of its prehistoric, protohistoric, ancient, and medieval descendents. Only rarely has it persisted in Indo-European languages to the modern day. It survived in Proto-Germanic in the first and second person pronouns, where it was then inherited by Old English, Old High German, Old Low German, Early Old Swedish, Old Norwegian, Old Icelandic, and Gothic. It continued in Icelandic until the 1700s, some dialects of Faroese until at least the late 1800s, and some dialects of North Frisian through the 1900s. From Proto-Greek it entered Ancient Greek, and from Proto-Indo-Iranian it entered Sanskrit. From Proto-Slavic, it still exists today in Slovene and the Sorbian languages. Indo-European languages that have long ago lost the dual still sometimes have residual traces of it, such as the English distinctions both vs. all, either vs. any, and neither vs. none. The Norwegian både, cognate with English both, has further evolved to be able to refer to more than two items, as in både epler, pærer, og druer, literally "both apples, pears, and grapes."

Trial

The trial number denotes exactly three items. For example, in Awa:

iya"dog"
iyatade"two dogs"
iyatado"three dogs"
iyamadi"dogs"

It is rare for a language to mark the trial on nouns, and some sources even claim that trial marking on nouns does not exist. However, it has been recorded for a few languages; besides Awa, Arabana, Urama, and Angaataha have trial number. It is much more common for a language to have trial pronouns, the case for the Austronesian languages of Larike, Tolai, Raga, and Wamesa. A minimal example is Nukna, which has only a single trial pronoun, nanggula, which can be either second or third person. The trial may also be marked on verbs, such as in Lenakel.

	Singular	Dual	Trial	Plural
First	aʔu	arua	aridu	ami
First		itua	itidu	ite
Second	ane	irua	iridu	imi
Third	mane	matua	matidu	mati

While the dual can be obligatory or facultative, according to Greville Corbett there are no known cases of an obligatory trial, so the trial might always be facultative. However, languages may have both a facultative dual and a facultative trial, like in Larike, or an obligatory dual and a facultative trial, like in Ngan'gi.
Most languages with a trial are in the Austronesian family, and most non-Austronesian languages with a trial are nearby in Oceania. The latter category includes the Austronesian-influenced English creole languages of Tok Pisin, Bislama, and Pijin. In Australia, the trial can also be found in Aboriginal languages of many different language families. In Indonesia, trial pronouns are common in the storytelling of Abun, a possible language isolate. In the Solomon Islands, trial pronouns are used very frequently in Touo, either a Central Solomon language or a language isolate. As a result, bilingual speakers of Touo and Pijin will use trial pronouns a lot more commonly in Pijin than other speakers, for whom the trial is usually a lot less common than the dual. A very rare example of a spoken language with the trial outside of Oceania is Muklom Tangsa, spoken in northeast India.