What are diagrams for? Look around the world and you will see diagrams everywhere. Not just in newspapers, on screens, in manuals, in textbooks, but on the very streets that we travel: road signs, zebra crossings, street dividers, parking lines, bicycle and bus lanes. The world we inhabit is not just designed, it is diagrammed. The ancient world displayed diagrams as well, with schematic maps in petroglyphs scattered across the world (see, for example, Brown 1979): a painting from the tomb of the Egyptian king, Rameses III, showing the making of bread step by step (Redford 2001), and a string of footprints carved in a stone walkway in Ephesus directing ancient Romans to the brothel. In all these cases, ancient and modern, diagrams select and visualize the information that is relevant, sometimes exaggerating and distorting it, and eliminate the information that is not relevant. The ancient examples capture the two fundamental kinds of information that diagrams convey: the static structural spatial information of a map, and the dynamic functional temporal information of instructions. Space and time, and then causality. Space, whether maps of towns, or of the bones in our bodies, or the structure of a corporate organization; time, whether a scroll depicting an emperor’s journey, or the steps in making of bread, or the narrative of a graphic novel.