The first setting starts off in two voices, and adds just one part in running quavers below the right hand’s plainsong semibreves; a third voice joins in as the music breaks into triple rhythms. This piece contains a 3-bar quotation (bars 42 to 44) from a Gloria tibi trinitas setting by one of the Chapel royal organists, John Blitheman. Even though Byrd chose not to follow his path stylistically as a composer, they were later to become colleagues for twenty years and presumably had a good professional relationship.
The second setting, in three voices, places the chant an octave lower (where it has to be played mostly by the thumb of the left hand). The quaver counterpoint is above, now, and a new one in minims is added, below. These soon change places, the minims give way to crotchets, the rhythm speeds up, and finally hints of a fourth voice are heard as the music heads for its cadence with considerable assurance.
from notes by Davitt Moroney © 1999