The medieval history of the Teutonic Knights can be gathered from a relatively rich corpus of materials. One of the most important collections is in the Geheimes Staatsarchiv Preußischer Kulturbesitz Berlin, XX. Hauptabteilung, but there are only partly printed calendars and repertories. This holds especially true for the registers and books in the group of the 'Ordensfolianten'. This book calendars the oldest registers with letters from the Grand Masters which allow insights in different aspects of the Order's history.