While the comparative study of the Office is supported by advanced projects such as CAO-ECE or Cantus, for example, there were no similar initiatives in the field of the Mass. It is not a historical accident: Mass repertories are normally more uniform than those of the Office and apparently offer less for empirical observations for an inquiry directed towards the regional and local variety of plainchant repertories. Closer inspection, however, shows that also in the field of the Mass:
Consequently, it seemed reasonable to work out a system that is able to compare the repertory of different Mass sources in a similar way to CAO-ECE in the field of the Office. Since Mass sources may be very different in size and content, in the arrangement of their material, in using written and unwritten rules how to celebrate the liturgy, in order to be able to compare them we need:
We followed CAO-ECE in seeking to make the inventories suitable for comparison and in trying to define source families or traditional repertories, but we used a different principles in the formation of the numbering system. While CAO-ECE uses continuous numbering (quite adequate for the Office structure), the NUMERUS of the Gradualia system is a composite of the numbering of the Tempus, the Dies, the Ferial days and the Function (genre) (see: Principles of GRADUALIA).
The principles of the project were worked out by Gábor Kiss and first introduced in detail at the 15th meeting of the IMS Study Group Cantus Planus in 2009:
The GRADUALIA homepage contains detailed inventories of Mass sources (Graduals as well as Missals). The sources were scrutinized according to a liturgical framework, expressed by a fixed numbering system. Since this arrangement is not always identical with the actual layout of the sources, the sources can be viewed basically in two forms: according to the formalized liturgical system, on the one hand, and according to the original order of the content, on the other.