Metonymy designates a relation based on combination; modes can be metonymically related when they are linked by an association, as when lines from a poem are combined with a melody from a song. It is a relation based on contiguity between elements in different modes.
Sorapure shares various student examples to illustrate how she uses the tropes in assessing multimodal compositions. She carefully demonstrates how metaphor and metonymy activate strong or weak relations between visual and verbal modes. (For example, if the composition lacks metaphor…it falls flat.)
While “text, sound, and image each add their own part to the meaning….” it is also crucial to look at the relations between modes -”because metaphor and metonymy designate relations between two or more entities, they can be used to describe the relations between modes.