This article is intended as a reference guide to various notions of monoidal
categories and their associated string diagrams. It is hoped that this will be
useful not just to mathematicians, but also to physicists, computer scientists,
and others who use diagrammatic reasoning. We have opted for a somewhat
informal treatment of topological notions, and have omitted most proofs.
Nevertheless, the exposition is sufficiently detailed to make it clear what is
presently known, and to serve as a starting place for more in-depth study.
Where possible, we provide pointers to more rigorous treatments in the
literature. Where we include results that have only been proved in special
cases, we indicate this in the form of caveats.