The famous story says Count Keyserlingk, a sleepless Russian diplomat in Saxony, asked Bach for keyboard music that his 14-year-old servant Johann Gottlieb Goldberg could play through the night, and rewarded him with a golden goblet holding 100 louis d'or. This episode explains why historians doubt that tale, from the 60-year gap before Forkel's 1802 biography to the absence of any dedication on a title page Bach paid to have engraved on copper and published himself in Nuremberg.

The episode then opens up the architecture: an opening sarabande aria, 30 variations built on its bass line and harmonic progression rather than its melody, and a return of the aria. It walks through the strict rule of three, where every third variation is a canon at a progressively wider interval from the unison to the ninth, followed by a genre piece and a hand-c ...