[...] 'functional' grammar […] is functional in three distinct although closely related senses: in its interpretation (1) of texts, (2) of the system, and (3) of the elements of linguistic structures. (1) It is functional in the sense that it is designed to account for how the language is 'used' [...] it is the uses of language that, over tens of thousands of generations, have shaped the system. Language has evolved to satisfy human needs; and the way it is organized is functional with respect to these needs – it is not arbitrary. A functional grammar is essentially a ʻnaturalʼ grammar, in the sense that everything in it can be explained, ultimately, by reference to how language is used.
- Halliday (1985), a pag.XIII

[...] each 'element' in a language is explained by reference to its function in the total linguistic system. In this third sense [functional grammar of the elements of linguistic structures], therefore, a functional grammar is one that construes all the units of a language – its clauses, phrases and so on – as organic configurations of functions. In other words, each part is interpreted as functional with respect to the whole.
- Halliday (1985), a pag.XIII-XIV

The relation between the meaning and the wording is not, however, an arbitrary one; the form of the grammar relates naturally to the meanings that are being encoded. A functional grammar is designed to bring this out; it is a study of wording, but one that interprets the wording by reference to what it means.
- Halliday (1985), a pag.XVII

[...] there is no clear line between semantics and grammar, and a functional grammar is one that is pushed in the direction of the semantics.
- Halliday (1985), a pag.XIX

The fact that this is a ʻfunctionalʼ grammar means that it is based on meaning; but the fact that is a ʻgrammarʼ means that it is an interpretation of linguistic forms. 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 [...] The relation between the semantics and the grammar is one of realization: the wording ʻrealizesʼ, or encodes, the meaning. The wording, in turn, is ʻrealized byʼ sound or writing. There is no sense in asking which determines which; the relation is a symbolic one. It is not possible to point to each symbol as an isolate and ask what it means; the meaning is encoded in the wording as an integrated whole. The choice of a particular item may mean one thing, its place in the syntagm another, its combination with something else another, and its internal organization yet another. What the grammar does is to sort out all these possible variables and assign them to their specific semantic functions.
- Halliday (1985), a pag.XX