passacaglia explained
formpass-ah-KAL-yah
A compositional form built on a repeating bass line or harmonic progression, typically in triple metre and minor key
The passacaglia originated as a Spanish dance but evolved into one of the most serious forms in Western music. Unlike a simple ground bass, the passacaglia's repeating pattern may migrate from the bass to upper voices. Bach's Passacaglia in C minor for organ is the supreme example: a 20-note bass theme supports 20 variations of increasing complexity before launching into a double fugue. The form was revived in the 20th century by Webern, Britten, and Shostakovich.
Britten's Peter Grimes contains a passacaglia that is widely considered the finest example of the form in 20th-century opera.