William Mahrt has written a commentary:
This simple, but pivotal text, from Isaiah is found on the Sunday immediately predeeding Christmas, culminating the Advent build-up in anticipation of the birth of the Lord. This Sunday in the earliest sources of the Roman liturgy was designated dominica vacat—because of ordinations held on the vigil of the Sunday, there was no liturgy on this Sunday morning and thus no propers were provided for the fourth Sunday.
Rather, the chants subsequently assigned to this Sunday were originally for Wednesday in Ember Week in Advent, the previous Wednesday. They also are found on the Annuniation in one early source. The link with the Annunciation is significant, because sometimes, it was actually observed in Advent.
Isaiah’s prophesy in this chant consists of only eleven words which fall into a typical bi-partite format, marked in modern editions by the colon. Yet, more importantly, the rhetorical structure of the text is in three parts, each subsequent part introduced by the conjunction “et.”
This rhetoric is well represented musically: the first phrase, “Ecce virgo concipiet,” outlines the fifth above D, touching upon the note below, and cadences on D. The second, “et pariet filium,” is quite distinct from the first, rising a whole octave to the high d and cadencing on a. Johner observes that there is a pentatonic element here, pointing out that “pariet” contains no half steps, but outlines the notes of the pentatonic scale DEGAC. Indeed, the whole first half of the melody is very sparing of the half step: “concipiet” touches the half step only at its end, as does “pariet filium.”
This first half of the piece is a good example of the use of alternate chains of thirds: the basic strong notes of the mode, D-F-a-c are alternated with another chain of thirds, C-E-G(-B); the beginning of “concipiet” is on a C-E-G triad, which then returns to F and D.
This segment of the phrase increases the intensity by expanding the range by a note below and a note above. The E and G are significant notes in “et pariet,” where the quick ascent through some skips makes that phrase more intense than the previous one. The number is unusual skips in these two phrases is somewhat high and establishes a melodic style that is rising, skipping, and low in half-step progressions.
Then comes the culminating phrase of the piece, “et vocabitur nomen ejus Emmanuel.” The half steps seems to have been saved for this phrase, for it begins with the reiteration of the alternation of b-flat with a; “nomen ejus” begins with the half step F-E and rises to a-b-flat-a, “Emmanuel” contains three E-F half steps.
Thus the three phrases of the piece represent a rhetorical progression: “Behold, a virgin shall conceive,” to a moderate but increasing melodic development; “and bear a son,” raising the effect of the phrase by rising to the highest pitch of the piece and cadencing on the fifth degree; “and his name shall be called Emmanuel” then adds a warmer and more colored sonority through more frequent half steps on the culminating designation of the name of the Savior.
Each of these advances supports an increasing sense of anticipation, which is appropriate for the last piece of the last Sunday before Christmas.
