Portolan charts were the first nautical charts of great practical precision for Mediterranean navigation, focused on coastlines, ports, and compass bearings. Their extraordinary coastal realism — still not fully explained by historians — changed the relationship between map and navigation. They were operational instruments kept as state secrets by the Genoese, Venetian, and Majorcan workshops. They represent the decisive shift from bookish or encyclopedic cartography to cartography in service of the professional navigator.