when wolfgang schmieder created the chronology for the bwv catalogue of bach's works in the 1950s, the assumption was that bach's musical output matched his responsibilities in each of the three distinct phases in his career: the period 1700–1717 when he was organist at lüneburg, arnstadt, mühlhausen and weimar; the period 1717–1723 when he was capellmeister at cöthen; and the period from 1723 onwards when he served as thomaskantor in leipzig. accordingly, the chamber music works by bach were automatically assigned to the cöthen period. later generations of bach scholars have recognized that bach's involvement with chamber and orchestral music continued in leipzig, especially through the collegium musicum; and accordingly schmieder's rigid chronology is no longer generally accepted. nevertheless, even though there is no direct confirmation for the dating of bwv 1014–1019, bach scholars agree that the circumstances surrounding the 1725 source probably point to the first versions of these sonatas being composed between 1720 and 1723 during bach's last years in cöthen. in the 1958 neue bach-ausgabe edition, the editor rudolf gerber was unaware that the 1725 manuscript had been largely copied by bach's nephew, who was only a pupil at the thomasschule at the time. in addition two of the three last movements in the sixth sonata copied by bach himself were borrowed from the sixth keyboard partita bwv 830, movements also included in the 1725 notenbüchlein for bach's wife anna magdalena bach. this suggests that the initial collection of sonatas, assembled for an unknown purpose, was probably copied from pre-existing compositions and hastily completed. this hypothesis is not only compatible with bach's heavy compositional duties as thomaskantor at the start of his period in leipzig; but also agrees with the dating of the sonatas to cöthen by bach's biographer johann nikolaus forkel: a letter to him in 1774 from bach's son carl philip emmanuel describes the sonatas as being 50 years old.