A constraint proposed by Ronald Langacker which places limits on how the theory of cognitive grammar operates. This requirement holds that the only entities permissible within the grammar of a language are: (1) phonological, semantic and symbolic units; (2) the relations that hold between them; and (3) the schemas that represent these units. This requirement excludes abstract rules from the model, in contrast to theories of grammar in formal linguistics.

Instead, knowledge of any given linguistic pattern is conceived in terms of a schema.