Part IV. External Form of Sacred Composition
10. The different parts of the mass and the Office must retain, even musically, that particular concept and form which ecclesiastical tradition has assigned to them, and which is admirably brought out by Gregorian Chant. The method of composing an introit, a gradual, an antiphon, a psalm, a hymn, a Gloria in excelsis, etc., must therefore be distinct from one another.
11. In particular the following rules are to be observed:
The Kyrie, Gloria, Credo, etc., of the Mass must preserve the unity of composition proper to the text. It is not lawful, therefore, to compose them in separate movements, in such a way that each of these movements form a complete composition in itself, and be capable of being detached from the rest and substituted by another.
In the office of Vespers it should be the rule to follow the Caeremoniale Episcoporum, which prescribes Gregorian Chant for the psalmody and permits figured music for the versicles of the Gloria Patri and the hymn.
It will nevertheless be lawful on greater solemnities to alternate the Gregorian Chant of the choir with the so called falsi-bordoni or with verses similarly composed in a proper manner.
It is also permissible occasionally to render single psalms in their entirety in music, provided the form proper to psalmody be preserved in such compositions; that is to say, provided the singers seem to be psalmodising among themselves, either with new motifs or with those taken from Gregorian Chant or based upon it.
The psalms known as di concerto are therefore forever excluded and prohibited.
In the hymns of the Church the traditional form of the hymn is preserved. It is not lawful, therefore, to compose, for instance, a Tantum ergo in such wise that the first strophe presents a romanza, a cavatina, an adagio and the Genitori an allegro.
The antiphons of the Vespers must be as a rule rendered with the Gregorian melody proper to each. Should they, however, in some special case be sung in figured music, they must never have either the form of a concert melody or the fullness of a motet or a cantata.