What we know as ʻgrammarʼ is the linguistic device for hooking up together the selections in meaning which are derived from the various functions of language, and realizing them in a unified structural form.
- Halliday (1973), a pag.42

[...] grammar: a level of organization intermediate between content and expression, which can take the various functionally distinct meaning selections and combine them into integrated structures. The components of the grammatical system are thus themselves functional; but they represent the functions of language in their most generalized form, as these underlie all the more specific contexts of language use.
- Halliday (1973), a pag.67

[...] Grammar is what the speaker CAN SAY, and is the realization of what he MEANS.
- Halliday (1973), a pag.82

[...] the term ʻgrammarʼ being used, as always, for the level of linguistic form, including both grammatical (in the sense of syntax and morphology) and lexical features. Grammar is the level at which the various strands of meaning potential are woven into a fabric; or, to express this non-metaphorically, the level at which the different meaning selections are integrated so as to form structures.
- Halliday (1973), a pag.92-93

Grammar is the level of formal organization in language; it is a purely internal level of organization, and is in fact the main defining characteristic of language. But it is not arbitrary. Grammar evolved as ʻcontent formʼ: as a representation of the meaning potential through which language serves its various social functions. The grammar itself has a functional basis.
- Halliday (1973), a pag.98