Langacker develops the central ideas of Cognitive Grammar in his seminal, two-volume Foundations of Cognitive Grammar, which became a major departure point for the emerging field of Cognitive Linguistics. Cognitive Grammar treats human languages as consisting solely of semantic units, phonological units, and symbolic units (conventional pairings of phonological and semantic units). Like Construction Grammar, and unlike many mainstream linguistic theories, Cognitive Grammar extends the notion of symbolic units to the grammar of languages.