Following the guiding thread of Peirce’s use of diagrammatic syntax in his system of existential graphs (EG), which depends crucially on the role of the Sheet of Assertion, we introduce the notion of Sheet of Indication (SI) as the basis for a general diagrammatic semantics applicable to a wide range of diagrams. We then show how Peirce’s EG-alpha graphs may be understood as instances of SIs and how logically coherent models of the graphs are represented in the SI semantics.