This book constitutes the refereed proceedings of the 9th InternationalConference on the Theory and Application of Diagrams, Diagrams 2016,held in Philadelphia, PA, USA, in August 2016. The 12 revised full papers and 11 short papers presented together with 5 posters were carefully reviewed and selected from 48 submissions. The papers are organized in the following topical sections: cognitive aspects of diagrams; logic and diagrams; Euler and Venn diagrams; diagrams and education; design principles for diagrams; diagrams layout.

Mathematicians at every level use diagrams to prove theorems. "Mathematical Reasoning with Diagrams" investigates the possibilities of mechanizing this sort of diagrammatic reasoning in a formal computer proof system, even offering a semi-automatic formal proof system--called Diamond--which allows users to prove arithmetical theorems using diagrams.