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From Citizendium, the Citizens' Compendium
Noun class refers to a system which categorises the nouns of a language into different groups. Nouns may be assigned to a group according to some semantic feature, i.e. they mean similar things; according to pronunciation; or arbitrarily. For example, the French language divides nouns into two classes according to an extensive set of rules relating to the form of each noun: for example, nouns ending -age almost always belong to one class, perhaps misleadingly named 'masculine'.
While noun classes originate in various distinctions found in the natural world, e.g. animacy (living and non-living) and gender, they are not typically filled according to whether things in the speakers' environment are obviously male, female, living or non-living, and so on. This means that while, for example, the noun man will usually be found in a class that includes recognisably 'male' things, or is associated with maleness in some way, other nouns will not be placed in their categories according to such basic associations. Looking at French again, while it is true that homme 'man' is assigned to the 'masculine' class, it is not the case that other masculine nouns such as lit are strongly associated with maleness. Often categorisation is not what would be expected: for example, sein, 'breast', is also masculine in French, while in German 'girl', Mädchen, is neuter.
Grammatical 'gender' and other sorts of classes
Typically, European languages have two or three noun classes, which are usually called 'genders' and referred to as 'masculine', 'feminine' and 'neuter'. As these words are related to the concepts of 'male' and 'female', such terminology may misleadingly imply that languages can only have up to three genders, and that each noun is assigned to a 'gender' according to how 'male' or 'female' it seems to be. In fact, many languages have far more than three classes, and often assign nouns quite specifically and in ways that reveal how the mind or cultures categorise the world. For example, the traditional four-class Australian Aboriginal language Dyirbal famously assigns for example 'women', 'fire', 'sun', 'watery objects' and 'dangerous things' to a single noun class named balan, while men as well as for example the moon, rainbows, fishing objects and some animals such as kangaroos and snakes are assigned to a different class named bayi. The third noun class named balam includes all edible fruits and vegetables, while bala includes only trees.
A noun's class can be explicitly indicated in various ways, depending on the language. For example, Italian singular masculine nouns usually end with -o, while feminine ones end with -a. In this case, nouns are divided primarily according to the morphology of the word, i.e. their form, with their final affixes (here, suffixes) denoting class. In French, native speakers can also identify the class of a noun according to its morphological form, with endings such as -age (masculine) associated with one class rather than another in most cases. Typically, there will be many exceptions to these rules, e.g. Italian aroma ('flavouring', 'aroma') is masculine despite the -a ending. Other words such as definite articles may 'agree' with the noun, with their forms changing according to the class: la città ('the city', feminine) and il telefono ('the telephone', masculine) exemplify this in Italian. In other languages, prefixes or infixes rather than suffixes may be used, with the an extra morphological unit added to the front of the word or inserted within it.
Many languages do not truly classify nouns, though most, if not all, seem to divide up nouns in some respect. English retains the basic animacy and gender distinctions in its pronouns: he and she are distinguished by gender, and in turn contrast with it in animacy. A more complex, vestigial appearance of noun classification occurs in nominal 'classifiers', which are forms that group a subset of nouns by meaning. For example, sheet acts as a classifier in sheet of paper - it is not possible to simply refer to *a paper with this meaning. However, nouns like paper are not contrasted with others in this way in all possible contexts, so this is not an example of a true noun class system at work in English. Other languages, such as Japanese, have a far more extensive network of classifiers. For example, in Japanese counting, ichiwa includes the -wa classifier specifically for counting birds, and ichi means 'one'. Ichiwa therefore means 'one bird'. Two and three birds would be niwa and sanba.