Halliday & Matthiessen (2004: 646):
… the general tendency for ideational metaphor is to ‘downgrade’ the domain of grammatical realisation of a semantic sequence, figure or element — from clause nexus to clause, from clause to group/phrase, and even from group or phrase to word. Such downgrading affects both the unit whose domain of realisation is downgraded, and the units of which it is composed: the downgrading proceeds down the rank scale by a kind of ‘domino effect’.