Systemic Functional Linguistics models language as a tri-metafunctional system comprising experiential, interpersonal, and textual domains.

In Introduction to Functional Grammar, co-authored with M.A.K. Halliday, Matthiessen details how clauses simultaneously enact three distinct metafunctions. The experiential metafunction construes our internal and external reality; the interpersonal enacts social relationships and speech roles; and the textual organizes these into a coherent flow of information. This tri-metafunctional architecture allows linguistic structures to be analyzed as multi-layered configurations, where grammatical elements like the Subject and Finite serve distinct functional roles across different systemic domains.