The “Kyrie” section of a mass comes first and must make an impression. Composers of ages past had learned to write a contrasting phrase in the middle: “Kyrie eleison, Christe eleison, Kyrie eleison.” Bach takes this to an extreme; two grand archaic Kyries frame a stylish, Italian opera style soprano duet. It couldn’t be more opposite. 

 

The beautiful intervals of thirds and sixths make the duet harmony, except when a special moment is called for, when we hear tense seconds and sevenths. But everything has its opposite, even within this duet. Thirds later become sixths, and seconds sevenths. Bach uses these relationships to balance the structure of the delicate prayer “Christ Have Mercy.”

 

Christe Eleison (Netherlands Bach Society):

https://www.youtube.com/watch?v=3FLbiDrn8IE&t=667s

 

Renaissance Kyrie example: William Byrd, Mass for Four Voices (Ensemble Morales)

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