Every distinction that is recognized in the grammar – every set of options, or ʻsystemʼ in systemic terms – makes some contribution to the form of the wording.
- Halliday (1985), Pag. XX

The organizing concept is that of the ʻsystemʼ which is used essentially in Firth’s sense of a functional paradigm but developed into the formal construct of a ʻsystem networkʼ. A system network is a theory of language as choice. It represents a language, or any part of a language, as a resource for making meaning by choosing. Each choice point in the network specifies (1) an environment, consisting of choices already made, and (2) a set of possibilities of which one is (to be) chosen; (1) and (2) taken together constitute a ʻsystemʼ in this technical sense.
- Halliday (1985), Pag. XXVII