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The Kalendarium Regine, composed by the French astronomer William of Saint-Cloud in 1296 for Queen Marie of Brabant and translated into old French as Kalendrier la Royne some years later, is an impressive document which illustrates the development of medieval sciences from a pure clerical culture to a lay culture. The Kalendrier reveals the significance of medieval calendars linking science and daily use not only by reflecting the state of knowledge in the field of astronomy and calendar computation around 1300 but also by a detailed didactic commentary which serves as an instruction manual.