Halliday & Matthiessen (2014: 661, 662):
However, even though we have to leave open the question of whether there is a single semantic compositional scale or more than one, we can say something about the lower region of such a scale or such scales. This is the region where semantic units are realised by lexicogrammatical ones. We have already referred to these semantic units throughout the book, and the principle that emerges here is the familiar metafunctional one. The clause is a multifunctional construct in the grammar, one that realises three different semantic units, one for each metafunction: textual – message, interpersonal – proposition/proposal, and experiential – figure. These relationships are shown in Figure 10-1: the three semantic units deriving from the three metafunctions are all mapped onto the clause, which thus unifies the three metafunctional strands of meaning.
