The plainchant melody at the root of this lovely motet is sometimes identified with the tune name AMBROSE. That is because the text of the Advent hymn Conditor alme siderum — sung in English as “Creator of the stars of night” — was long attributed to St. Ambrose (339-397). (You can see the music and hear that melody chanted by Cistercian monks here.)
As is the case with many other Renaissance-era adaptations of plainchant tunes, Palestrina’s motet alternates between plainchant and intricate polyphony. Let’s consider the opening measures to get a sense of what Palestrina is up to.
The opening words, “Conditor alme siderum,” are chanted by tenors, then the altos enter with the second melodic phrase of the traditional chant, but slowed down to about half-speed, and a perfect fifth higher than where the tenors left off. As the altos sing the second note of the phrase, the sopranos enter with the second phrase, also slowed down, and this time, an octave above where the tenors left off. When the sopranos get to the fourth note in the phrase, the basses enter with their presentation of the second phrase, imitating what the sopranos just sang. By this time the altos are flying off on a faster variation of the melody, followed soon by the sopranos. The tenors finally come in with their iteration of that second phrase, hitting their first note just as the basses get to the last note of the phrase.
Then the third phrase of the melody shows up, first (slowed down) sung by the tenors, quickly followed by altos and sopranos, and finally the basses. Fragments of the melody keep showing up throughout the first stanza. In the last 6 measures, all four parts take a stab at singing the final phrase of the melody, with slight modifications. Then the second stanza is chanted, followed by the third stanza sung again in four parts, but this time there are two tenor parts and no bass part, allowing for a slightly higher sound. The plainchant melody
The fourth stanza is chanted followed by a five-part fifth stanza; the basses come back joining the two tenor parts. The final stanza is a Doxology: “Praise, honor, power, and glory be to God the Father, and the Son, and likewise to the Holy Spirit unto ages of ages. Amen.” It is sung with six parts (soprano, two altos, two tenors, and bass), which gives the richest and fullest sound we’ve heard yet, and all six parts get a turn at singing fragments of the plainchant melody, all interwoven into a remarkable tapestry of sound.
You may download the score to this motet here. Below is a performance by the Belgian early music group, the Huelgas Ensemble, conducted by Paul Van Nevel.