Costituents in speech and writing both express and symbolize this more abstract structural order. At the same time, each of these hierarchies is indipendent of the others, and there are infinite possibilities of matching them up in meaningful way. They can be played with, as it were – precisely because, in the last resort, they are all on the surface of language. They are not what language really is – though they are essential mechanism for achieving its variety of purposes.
- Halliday (1985), a pag.18