Halliday & Matthiessen (1999: 68):
Here meaning is construed in terms of a spatial metaphor; we can view the ideation base as an elastic, multidimensional semantic space. […] The notion of a vowel space (with it ‘cardinal vowels’) provides a familiar analogy. As a material construction, it is limited to the three dimensions of physical space; but as physiological space, it accommodates variation along a number of dimensions, and brings an elasticity to the expression plane that is in some extent analogous to the metaphorical elasticity that we are ascribing to the plane of the content.