Halliday & Matthiessen (1999: 605-6):
… the system of phonology realises that of lexicogrammar; the system of lexicogrammar realised in phonology realises that of semantics; the system of semantics realised in lexicogrammar realised in phonology — which we call “language” — realises the system of context (i.e. the “culture”, considered as a semiotic potential).