Halliday & Matthiessen (1999: 106, 107):
We can explore this distinction through the conventions of comic strips. Expansions are typically represented pictorially by means of consecutive frames — 'x then y', 'x so y', 'x meanwhile y', and so on (the relationship is sometimes spelt out in writing, e.g. Meanwhile... ).
We can even identify the three different kinds of expansion, those of elaborating, extending, and enhancing. When further detail is given in a picture of a magnifying glass, this is elaboration; where two frames are joined to form a continuous picture, this suggests extension; otherwise, consecutive frames display a relation of enhancement But in all cases the representation is pictorial throughout.
In contrast, projections involve two modes: the projecting figure is represented pictorially as the symbol source, but the projected figure is represented linguistically and this second-order reality is typically named within a 'balloon'. … The two types of relation can be shown as in Figure 3-1.
