Halliday & Matthiessen (1999: 294-5, 296):
The whole metaphorical elaboration is made possible by a fractal pattern that runs through the whole system. We have suggested that the metaphorical elaboration is a token-value relation; but in order for it to be a token-value relation within the semantic system, it has to be natural in the sense that the token and value domains have to be similar enough to allow the token to stand for the value. For instance, a sequence has to be similar enough to a figure to allow it to stand as a metaphorical token for this congruent value. The principle behind this similarity is the fractal pattern of projection/expansion that we met throughout the semantic system of sequences, figures, and elements.
That is, while grammatical metaphor constitutes a move from one "phenomenal domain" to another — from sequence to figure, and from figure to element, this move is made possible because the fractal types engender continuity across these domains: the metaphorical move from one phenomenal domain to another takes place within one and the same transphenomenal domain. For example, the metaphorical shift from he added and smiled to he added with a smile is a shift from the phenomenal domain of sequence to the phenomenal domain of figure (accompanied by the shift from figure and smiled to element with a smile)', but the transphenomenal domain of 'extension' remains constant: the extending sequence he added and smiled is metaphorically agnate with the figure he added with a smile with an extending circumstance. …
