Citation: Joyal, André, and Ross Street. "The geometry of tensor calculus, I." Advances in mathematics 88.1 (1991): 55-112.
Web: https://www.sciencedirect.com/science/article/pii/000187089190003P
Tags: Mathematical, Foundational
This paper is a first in a series of two articles. Together, these papers serve as early formal uses of string diagrams to express the structures of (tensor) categories. The other paper in the series is
> Joyal, André, and Ross Street. "The geometry of tensor calculus II." Unpublished draft, available from Ross Street's website 312 (1995): 313.
These papers make formal the diagrams that their contemporaries were drawing, and proves rigorous mathematical theorems about them. This is a good "original reference" for string diagrams in category theory.