In order to make explicit the fact that syntax and vocabulary are part of the same level in code, it is useful to refer to it comprehensively as ʻlexicogrammarʼ; but it becomes cumbersome to use this term all the time, and the shorter term [grammar] usually suffices.
- Halliday (1985), a pag.XIV

The existing interface, that between meaning and expression, was already arbitrary, or was becoming so in the later protolinguistic stage: there is no natural connection between the meaning ʻI want that, give it to meʼ and the sound 'mamama' or 'nanana' often produced by a ten-month-old as its realization. It was necessary for the system to develop this frontier of arbitrariness, otherwise communication would be restricted to the relatively small range of meanings for which natural symbols can be devised. But it was not necessary that the 'new' interface, that between meaning and wording, should become arbitrary; indeed there was every reason why it should not, since such a system, by the time it got rich enough to be useful, would also have become impossible to learn. Thus the lexicogrammar is a natural symbolic system [...] What it means is that both the general kinds of grammatical pattern that have evolved in language, and the specific manifestations of each kind, bear a natural relation to the meanings they have evolved to express.
- Halliday (1985), a pag.XVIII