The first of two volumes (the other being RCM42) including all the known music by an important but shadowy mid--fifteenth-century composer. According to the musicologist Reinhard Strohm, Petrus may be synonymous with Pierre Maillart, Burgundian singer and ecclesiastic. This elegant Mass is in three voices and at its written pitch would be suitable for two baritones and a high tenor. vi + 29 pages.