A set of linguistic forms to which it is typically more difficult for a language to add new members. Closed class forms are normally taken to include the 'grammatical' or 'function' words of a language.

In English these include articles, prepositions, pronouns, inflectional morphemes and so forth. In terms of the meaning contributed by the closed class elements they provide schematic meaning. They contribute to the interpretation of an utterance in important but often subtle ways, providing a kind of 'scaffolding' which supports and structures the content meaning provided by open class forms.