Because of its unusual tonal layout and movement structure, there has always been speculation that the last of J.S. Bach's six authentic sonatas for violin and harpsichord, the Sonata for violin and harpsichord No. 6 in G major, BWV 1019, is something of a hodge-podge, put together from existing movements that Bach already had lying around. While this may indeed have been the case, we do know that Bach himself revised the work on two occasions during the Cöthen years (1717 - 1723), each time radically changing the design of the work by substituting whole strings of movements for one another; so, ad hoc as the original construction may have been, he certainly spent time and energy making it into a piece of which he approved. Today, Bach's final version is considered definitive, though some feel that it is not the best version of the three.
Both the first and last versions of BWV 1019 are in five movements. In the original version three central slow movements are enclosed by two Allegro "bookends" -- the second of which is actually just the first replayed. The second and middle of these slow movements (Cantabile, ma un poco adagio) appears in neither of the later re-workings, but is among Bach's most eloquent lyrical statements. Two of the original movements -- the first and second, Allegro and an E minor Largo -- are used in the final version of BWV 1019. After this, Bach very surprisingly supplies a dense, binary-form Allegro in E minor for harpsichord solo in the central slot, and then a new Adagio third movement in B minor and a new Allegro finale. Two different movements for harpsichord solo, both dance-genre pieces and both eventually reused in the E minor Partita for keyboard, BWV 830, appear in the second version of the sonata.