The most important composers in Venice were two Gabrielis, Andrea (c. 1510–1586) and his nephew Giovanni. As organists of St. Mark’s Basilica, the cathedral of Venice, both of them exploited the special acoustics of that extraordinary building, which still impress tourists today. By placing choirs of singers and instrumentalists in some of St. Mark’s many different choir lofts, they obtained brilliant echo effects that even modern audio equipment cannot duplicate.

Giovanni’s “O magnum mysterium,” part of a larger motet, was written for the Christmas season. The words marvel that lowly animals — the ox and the ass — were the first to see the newborn Jesus. This naïve, touching text made “O magnum mysterium” a favorite for motet settings at the time; there are lovely versions by Victoria and Byrd (page 70).

Gabrieli’s music marvels along with the text. In the manner of a madrigal, the exclamation O is repeated like a gasp of astonishment. Then lush chord progressions positively make the head spin, as the words O magnum mysterium are repeated to the same music, but pitched higher (that is to say, in sequence — see page 48).

Gabrieli uses two choirs, each with three voice parts and four instrumental parts, plus organ, though at first all we hear is a sumptuous blend of brass instruments and voices in a mainly homophonic texture. A more polyphonic texture emerges for the first time at the word sacramentum. Solo voices, first tenors, then boy sopranos, imitate one another during the line iacentem in presepio. Their motive is finally taken up by the brass.

Gabrieli unleashes his musical resources in a big way at the choral “Alleluia” section. The music moves in quick triple meter, matching the jubilation of repeated alleluias, and the choirs echo back and forth across the sound space:

To make a grand conclusion, the two choirs come together again. There is another wash of voice-and-brass sonority as the tempo slows and the meter changes to duple for a climactic alleluia. And for still more emphasis, Gabrieli repeats the entire “Alleluia” section, both the fast triple-time alternations and the massive slow ending. This kind of clear sectional repetition shows one way Baroque composers worked to impose clarity and control on flamboyant chords and the solo rhapsodies.