Halliday & Matthiessen (1999: 218):
Earlier we tried to suggest how the circumstantial elements were related to participants. There, we were looking at them as oblique "cases", from the point of view of their function in the larger figure. Two points emerged: one, that participants and circumstances taken together formed a cline, rather than being separated by a clear boundary; the second, that some of the circumstantial elements could be 'paired off with participants, being seen as a more oblique manifestation of a similar role. We were able to incorporate these two points in a helical form of presentation in Figure 4-14.
