Being musically ignoraant, among other ways, I searched for "basso obstinato". Perhaps others
may forgive me for posting:

Ground Bass(It. Basso ostinato,`obstinate bass')


Short thematic motif in bass which is constantly repeated with changing harmonies while
upper parts proceed and vary. Originated in Cantus firmus of choral mus. and became popular
in 17th cent., particularly in Eng., as a ground for variations in str. mus. Hence the no. of
`Divisions on a ground'. Examples exist by Byrd, Purcell, Frescobaldi, Carissimi, and Cavalli. See
Chaconne.

I wonder what that girl thought "castrati" were, And what kind of mechanical devices she had in mind.