The sequence is:

a. a relatively short chant with a small number of phrases, often four, and arather narrow vocal range; they are invariably strophi
c.
b. an addition of music or text, or both, to a preexisting chant that more fullyexplains the theology of the chant to which they are added.
c. the first of the two melismatic, responsorial chants of the Proper of the Massthat are sung

between the Gloria and the Credo; it consists of two parts: arespond and a psalm verse.
d. a chant sung on high feasts during the Proper of the Mass immediately afterthe Alleluia, in which successive lines of text were paired into double verses.

d