It's often said informally that the Mass formularies, as in the Liber, with their assignments such as “For Double Feasts, 3” are not normative.
On the other hand Paschal and Requiem Masses are strongly associated with certain melodies: almost normative.
Is there any history to read about how the 18 (?) (or 16?) formularies in the Liber came to be assembled, and more to the point, the background and traditions of assigning particular ordinary chants to Masses and seasons?
My feeble understanding, based on vague recollections, leads me to surmize that the arrangement of mass ordinaries in LU was compiled by the monks of Solesmes in the later 19th c. as part of a systematization intended to provide a common practice throughout the Roman church. Its predecessor, the 1871 Putstet Graduale shares some commonalities but also considerable divergences. Putstet has 13 settings whereas LU has 18. I imagine that in the Solesmes researches they decided to add more settings to reflect the wide range of their available source material. What is different about Sarum, and probably other medieval liturgies is, as Andrew points out, that the ordinaries are not grouped by feast type; rather, all the kyries are given first, then the glorias, etc. with rubrics indicating when each chant is to be used. It would be a great study for some keen graduate student!
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