The diagrammatic mode is an important feature of medieval religious art. This book examines one of the most spectacular examples, the series of radial diagrams accompanying the manuscripts of ?De missarum mysteriis’ one of the most influential commentaries on the liturgy of the Mass, written in Rome between 1195 and 1197 by Lothar of Segni (ca. 1160–1216), shortly before his election as Pope Innocent III.