In linguistics, a closed class (or closed word class) is a word class to which no new items can normally be added, and that usually contains a relatively small number of items. Typical closed classes found in many languages are adpositions (prepositions and postpositions), determiners, conjunctions, and pronouns. Linguistics is the scientific study of language. ... In grammar, a part of speech or word class is defined as the role that a word (or sometimes a phrase) plays in a sentence. ... In grammar, an adposition is any of a wide variety of particles and affixes which are attached to a noun phrase (their object) in order to modify the noun phrase or show its relation to another concept or situation in the same clause. ... It has been suggested that this article or section be merged with adposition. ... A postposition is a type of adposition, a grammatical particle that expresses some sort of relationship between a noun phrase (its object) and another part of the sentence; an adpositional phrase functions as an adjective or adverb. ... Determiners are words which quantify or identify nouns. ... In linguistics and grammar, a pronoun is a pro-form that substitutes for a noun phrase. ...

Contrastingly, an open class offers possibilities for expansion. Typical open classes such as nouns and verbs can and do get new words often, through the usual means such as compounding, derivation, coining, borrowing, etc. In linguistics, an open class (or open word class) is a word class that accepts the addition of new items, through such processes as compounding, derivation, coining, borrowing, etc. ... In English, a noun or noun substantive is a lexical category which can co-occur with (in)definite articles and attributive adjectives, and function as the head of a noun phrase. ... This article or section is in need of attention from an expert on the subject. ... In linguistics, a compound is a lexeme (a word) that consists of more than one other lexeme. ... There are several meanings of derivation: A derivation in abstract algebra is a linear map that satisfies Leibniz law. ...

A closed class may get new items through these same processes, but the change takes much more time. The closed class is normally viewed as part of the core language and is not expected to change. Most readers no doubt can think of new nouns or verbs entering their lexicon, but it's very unlikely that they can recall any new prepositions or pronouns appearing in the same fashion.

References

• Dixon, R. M. W. (1977). Where have all the adjectives gone?. Studies in language, 1, 19-80.