Metonymy depends upon a principle of substitution, in which one word stands in the place of another on the basis of some material, causal, or conceptual relation. The plowman homeward plods his weary way, And leaves the world to darkness and to me. As he swung toward them holding up the hand, Half in appeal, but half as if to keep The life from spilling. . . . In all three examples, one finds a distinctly poetic expansion of meaning. In each case, something larger, vaguer, and more portentous stands in place of something that in itself might seem relatively trivial.
Poets can also achieve the opposite effect with metonymy.
Instead of expanding the significance outward, metonymy here contracts the focus to a few poignant details. By doing so, it lends the lines greater immediacy and specificity. This particular kind of metonymy, in which a part of something stands in place of the whole thing is often called synecdoche . The logic of the relationship between the terms is established, rather, by the metaphor itself. Some work in obvious ways, some do not.