Halliday (2008: 164-5):
I have used the term for three distinct dualities: first grammar and lexis, then system and text, and now speaking and writing. These three dyads are all distinct from each other. Grammar and lexis form a complementarity of focus, based on delicacy: meaning as generalities or as particulars. System and text form a complementarity of angle, based on instantiation: meaning as collections of instances or as potential. Speaking and writing are different from either of these. They form a complementarity of state, in the sense of different states of matter as solid, liquid and gas — except that here we are concerned with different states of meaning.