Sang or performed a descant (a high melodic line sung above the main melody); also means to discuss at length.
From 'descant' derived from Old French 'deschant' meaning 'a song or tune.' In music, it emerged in medieval times to describe the counterpoint sung above plainsong. The secondary sense of 'discuss' evolved metaphorically.
Medieval choirs invented descants to add complexity to simple church melodies—it's like they discovered that singing multiple melodies at once created something richer than any single voice could achieve.
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