Footnote 19

[19]The scene I've spun around this opening is of course imaginary, but it reflects the vividness with which Bach here portrays the pre-improvisational: the vacant noodling which may precede improvisation proper. A small but essential aspect of this portrayal is its suggestion that the right hand alone has initially been placed on the keys. Each of the piece's first two gestures includes a broken interval whose lower note is supplied by a flick of the right-hand thumb . In the ways of keyboard playing, such self-accompanying is something a right hand will resort to when the left is not participating.