This is, of course, a problem that is wider than the present document. Ever since Musicam Sacram (1967), the admission of alius cantus aptus, “the anthrax in the envelope” according to Lazlo Dobszay, any other suitable song in place of the proper chants, has meant in practice the virtual abandonment of the Gregorian propers.
The "anthrax in the envelope" was paragraph 32 of the 1967 Instruction. It says: In some places there exists the lawful practice, occasionally confirmed by indult, of substituting other songs for the Introit, Offertory and Communion chants in the Graduate Romanum. At the discretion of the competent territorial authority this practice may be retained, on condition that the songs substituted fit in with those parts of the Mass, the feast, or the liturgical season. The texts of such songs must also have the approval of the same territorial authority. (DOL 4153)
The General Introduction to the new Missal went a step further by saying that the Proper chant (Introit, Offertory, Communion) may be a piece from the Graduate Komanum or the Simple Gradual, — "vel alius cantus aptus" — or anything else that is appropriate. This sounds rather like the hoary joke about the causa bibendi, the rightful reasons for drinking: dies natalis, infirmitas corporis, adventus hospitis, vel alia causa - a birthday, bodily weakness, arrival of a guest, — or any other reason.
the hoary joke about the causa bibendi
Sometimes the practice was to sing the vernacular alongside (e.g. after) the Latin chant, other times it was a replacement of it.
(GIRM 48, emphasis mine)... (4) another liturgical chant that is suited to the sacred action, the day, or the time of year, similartly approved by the Conference of Bishops or the Diocesan bishop.
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