Halliday & Matthiessen (1999: 504):
A stratified semiotic defines three perspectives, which (following the most familiar metaphor) we refer to as ‘from above’, ‘from roundabout’, and ‘from below’: looking at a given stratum from above means treating it as the expression of some content, looking at it from below means treating it as the content of some expression, while looking at it from roundabout means treating it in the context of (i.e. in relation to other features of) its own stratum.