Between 1195 and 1197, Lothar of Segni composed a treatise devoted to the pontifical mass (De missarum mysteriis), which was soon schematically adapted in the form of diagrams. Among the few manuscripts containing these geometric diagrams, the Italian witness housed in Munich (BSB, Clm 28609) has captured our attention. A codicological study suggests a dating of 1220-1230, indicates that the quire of drawings was originally complete, and that it was probably meant to exist independently of the treatise. Whereas the first known manuscript, dated ca. 1200-1210 (Erfurt, Gotha, Memb. I 123), archors the invention of the drawings in Paris, a rebalancing towards Italy opens the possibility of other intellectual models : are they situated in the sphere of the Victorine school in Paris and its « visual exegesis », or in the Italian lineage of Joachim of Fiore ? As for the iconographic models of these diagrams, all the forms save one are centralized or circular, where we recognize variations on the motif of the Eucharistic host, specially adapted to a Eucharistic treatise. Of the twenty-seven diagrams remaining in Clm 28609, three are studied here in depth, from the perspective of Lothar’s treatise and with specific attention to the practical dynamics of reading that stimulate their exceptional layout.