Halliday & Matthiessen (1999: 5-6):
A word needs to be said here about the interpretation of the term "grammar" itself. As used in systemic theory, this term stands for lexicogrammar. The lexical region, or lexis, is not a separate component, but simply the most "delicate" end of the (unified) lexicogrammar. There is a complementarity here. Lexis and grammar are not two different phenomena; they are different ways looking at the same phenomenon. Some aspects of this phenomenon of "wording" in language are foregrounded by viewing it as grammar, others by viewing it as lexis.