Halliday & Matthiessen (1999: 69, 71):
We can now consider a comparable example from the content plane of language — from the ideation base. In our description of sequences, we recognised two simultaneous systems: the relative status of the figures ('equal/ unequal') and the kind of relationship between them ('projecting/ expanding'). This account constitutes the typological perspective on this region of the ideation base; it is mapped onto the topological perspective in Figure 2-9. Sequences are thus construed as a two-dimensional region within the overall semantic space. The correspondences are the same as those already noted for the vowel space.
