In the baroque period it was quite a common practice to rearrange works by other composers. Most of Bach's harpsichord concertos are arrangements of concertos by other masters (Vivaldi, Marcello etc.) or of works on his own wich were not even composed in concerto form in the firs place.
Thus for example the first and the third movement of the Triple Concerto have been formed - with a few alterations to suit the new medium - from the Prelude and Fugue in A minor for keyboard (BWV 894). In the second movement, on the other hand, the three solo instruments play almost note for the middle section of the Trio Sonata in D minor for Organ (BWV 527), transposed into C major.
As regards the Harpsichord Concerto No. 8, only the first nine bars, i. e. the opening tutti, were worked out by Bach (BWV 1059). All the rest has been highly successfully reconstructed by Gustav Leonhardt, a harpsichordist and musicologist of international repute on account of his performance of baroque music, with an extraordinary feeling for Bach's method of arrangement. The first nine bars seem to be a version of the opening of the first Sinfonia (for organ, strings and three oboes) to Cantata No. 35. This served as a pattern for the first movement of the restored concerto, for which Bach had required "Cembalo solo, una Oboe, due Violini, Viola e Cont.". The thematic material differing from this in the first nine bars has, also been incorporated into the original organ part, entirely on the model Bach has provided with the "harpsichordizing" of other works. It is also as good as certain that the other Sinfonia of the same cantata was to have become the last movement of our concerto. The orchestral parts have been taken over almost unchanged and the harpsichord part adapted to suit the instrument. The slow middle movement must, however, be regarded as lost. In order not to have to dare composing a complete imitation "Bachian" movement, Leonhardt has followed the example set by Bach himself in his Thrird Brandenburg Concerto. There he has only written two cadenza chords, everything else being left to the performer. In the same way a short improvisation leads into the last movement of this harpsichord concerto.

String Instruments in baroque dimensions: thinner bass bar, less slanting neck, thicker bridge. Gut strings, only lowest covered. Specific baroque tone-colour, of sonorous clarity.
Harpsichord on historical model with raven quill jacks. Pitch: German "kammerton" of the 18th century, nearly a semitone below, modern pitch.