Chants of the Vatican Graduale - MusicaSacra

4 Structure and Expressiveness of the Variable Mass-Chants
The fourth mode has the final cadence a gf ga g dgff when the antiphon
begins with df, as for instance on Maundy Thursday:
dfe
nos.
The first mode has the final cadence fff dcdf, when the antiphon
begins with cd, as for instance on the feast of the Purification:
cd dab\> a a
Su- see- pimus.
Otherwise it has the cadence fga, if the antiphon sets in on a, as,
for instance, on the feast of the Conversion of St. Paul:
a gag
Sci- o.
The fifth mode has the stirring and onward-urging cadence c d b c
dgdgfg, especially when the antiphon is defined within the tone range
f-a, as, for instance, on Septuagesima Sunday:
fa a a a agga f
Cir-cum-de -de - runt me.
The sixth mode has the still more impelling cadence fgfdfgf§
a g when the antiphon sets in on a low pitch, as, for instance, on Low
Sunday:
c d d d
Quasi mo-do
Evidently the need for contrast also comes into play here. The
"concors varietas," as St. Augustine 1 has it; the "suavis quaedam et concordabilis
diversitas—smooth and harmonious diversity," as Berno of
Reichenau 2 (eleventh century) puts it. The same becomes apparent from
the rule for the adaptation of individual phrases of the same composition:
if one phrase closes on a low pitch, the following will have a tendency to
rise. Thus in the Offertory of the second Sunday in Advent, the fourth
phrase begins with g-c, while the foregoing one closes with //. This is
seen in various Graduals of the third mode, for example, the close and
the beginning of the first and second phrases of Quinquagesima Sunday;
three times on the feast of the Holy Name of Jesus; twice on the feast
of St. Michael.
On the other hand, if one phrase begins in a high pitch, the following
will have a tendency to begin with a low pitch. This is well exemplified
in the Introit of the third Sunday of Advent, where the cadence over
1 De Civitate Dei, 1, 17, c. 14.
2 Gerbert, Scriptores eccles. de musica, II, 77.