What's a string diagram? These are the diagrams that are Poincare' dual to the usual: objects become 1-dimensional manifolds (strings) and arrows become 0-dimensional (nodes), as per (R. Street and A. Joyal) The geometry of tensor calculus I, Advances in Math. 88 (1991) 55-112 but with latest developments Tom brought back from a workshop with Street as one of the speakers