Comparison of a considerable number of Gregorian sources dating from the tenth to the thirteenth centuries reveals a handful of Mass Proper items that do not belong to the standard repertory and show possible ties with the Byzantine and/or Gallican traditions. These pieces are not recorded in most of the earliest French and German sources of the Gregorian tradition. Some of them seem to have been composed in Italy (but not in Rome), while others would appear to have Eastern or Frankish ties. Comparative melodic analysis, along with the discussion of their position in the liturgical year, discloses insights about their origin, date, routes of transmission and the ways to compare chants belonging to different liturgical families.