Halliday & Matthiessen (1999: 602):
It is not autonomous; it is itself part of a more complex semiotic construct — which … can be modelled in stratal terms such that language as a whole is related by realisation to a higher level of context (context of situation and of culture). This contextualisation of language … was the critical factor which made it possible to relate language to other systems–&–processes, both other semiotic systems and systems of other kinds.