A poem can travels through both vertical and horizontal levels of meaning by way of substitution. The poet can chose to substitute the literal for the figurative or the abstract or vice-versa. Substitution can even become a poet's overall strategy or "organizing principle" whether employed as a technique or happening unconsciously.
There are a number of poems offered in this section as examples, but the one poet whose work comes to my mind is Matthew Zapruder. "Global Warming" and "White Castle" are two fine examples when it comes to substitution and the movement of meaning within poetry.
A substitution exercise from Jack: "Find some literal images in your poem and then add an abstract quality or specific detail to those images.
Happy writing, Andrea