The concertos for two or more harpsichords date from a slightly earlier period. The parts from the Concerto for four harpsichords BWV 1065 (Bach's arrangement of the Concerto for Four Violins, RV 580, by Antonio Vivaldi), have been dated to around 1730. Whereas the harpsichord concertos were composed partly to showcase Bach's own prowess at the keyboard, the others were written for different purposes, one of them being as "Hausmusik"—music for domestic performance within the Bach household at the Thomasschule in Leipzig. Johann Nikolaus Forkel, Bach's first biographer, recorded in 1802 that the concertos for two or more harpsichords were played with his two elder sons. Both of them corresponded with Forkel and both remained in the parental home until the early 1730s: Wilhelm Friedemann departed in 1733 to take up an appointment as organist at the Sophienkirche in Dresden; and in 1735 Carl Philipp Emanuel moved to the university in Frankfurt to continue training for his (short-lived) legal career. There are also first-hand accounts of music-making by the entire Bach family, although these probably date from the 1740s during visits to Leipzig by the two elder sons: one of Bach's pupils, J.F.K. Sonnenkalb, recorded that house-concerts were frequent and involved Bach together with his two elder sons, two of his younger sons—Johann Christoph Friedrich and Johann Christian—as well as his son-in-law Johann Christoph Altnickol. It is also known that Wilhelm Friedemann visited his father for one month in 1739 with two distinguished lutenists (one of them was Sylvius Weiss), which would have provided further opportunities for domestic music-making. The arrangement of the organ sonatas, BWV 525–530, for two harpsichords with each player providing the pedal part in the left hand, is also presumed to have originated as "Hausmusik", a duet for the elder sons.