Hammer, Eric M. and Danner, Norman. 1996. “Towards a Model Theory of Diagrams.”The Journal of Philosophical Logic 25(5): 463–482. Reprinted in Allwein and Barwise (1996, 109–128).
Further References
Allwein, Gerard and Barwise, Jon K., eds. 1996. Logical Reasoning with Diagrams. Oxford: Oxford University Press.