Halliday & Matthiessen (1999: 30):
We have noted above that the semantics/lexicogrammar of natural language is itself a ‘realisation’ (an abstract construction) of daily experience. Likewise, the system we use to explore the semantics/lexicogrammar — our theory of semantics and our grammatics — is a ‘realisation’ of that part of daily experience that is constituted by semantics and lexicogrammar; that is, it is an abstract construction of language. This system is itself a semiotic one — a metalanguage; in Firth’s more everyday terms, it is language turned back on itself. So whereas a language is (from an ideational point of view) a resource for construing our experience of the world, a metalanguage is a resource for construing our experience of language.