Halliday & Matthiessen (1999: 173-4):
The "degree of involvement", in the sense of how deeply some element is involved in actualising the process that is construed by the figure, can thus be represented as a cline: the difference appears not only between participants and circumstances as a whole, but also within each of these primary categories, so that there is a continuum from one to the other along this scale.
At the same time, and cutting across this cline of involvement, we find that — like the participants themselves — the circumstantial elements fall into distinct types according to their relationship to the Process + Medium nucleus. These types correspond to the four transphenomenal categories of logico-semantic relations that are now familiar: the circumstance is either a circumstance of projection or a circumstance of expansion and, if the latter, then either elaborating, extending or enhancing. If we combine the degree of involvement with the logico-semantic categories, we can represent the elements of a figure in the form of a helix (Figure 4-14).
