One of the most important treasures of the Göttingen City Archives is a late medieval manuscript; it is the so-called Göttingen St. Johannis Missale. A missal contains the lectures assigned to the priest at the mass and all the songs assigned to the choir or the schola. Since the Göttingen city church, the Johanniskirche, is dedicated to John the Baptist, it was only natural to concentrate on the chants celebrated in his honor from the six masses included into the missal. All the lectures and other parts performed by the priest are substituted by instrumental music, played by the wind ensemble Aeolos on historical instruments. This is intended to clarify the dialogical principle that usually characterizes the celebration of the mass.