Halliday & Matthiessen (1999: 288):
The correspondence that is construed through grammatical metaphor is an elaborating relationship: an identity is set up between two patterns … In this identity, the metaphorical term is the ‘Token’ and the congruent term is the ‘Value’ … The identity holds between the two configurations as a whole; but … the components of the configurations are also mapped one onto another …The metaphorical relation is thus similar to inter-stratal realisation in that it construes a token–value type of relation. Here, however, the relation is intra-stratal: the identity holds between different meanings, not between meanings and wordings. The metaphor consists in relating different semantic domains of experience …