O MAGNUM MYSTERIUM is divided into three parts: the first, in a major key, with the four voices that combine, follow, and chase one another to announce the great mystery of the birth of Christ. The second part is in a minor key, and has a darker, melancholy color, and introduces us to the third, conclusive section. The final Alleluja is in mixolydian mode, and has a triumphant and luminous character, but the brief coda brings us back into a more intimate and tranquil climate.