This is a duet arrangement of Bach’s adaptation of the Violin Concerto in C major op 1 No.4 by Johann Ernst Prinz von Sachsen Weimar. However, the original does not belong to the set of Johann Ernst’s concertos edited by Telemann and published in 1718. Its attribution is based only on the title, and the original has not been found. Johann Ernst of Saxe-Weimar (1696 –1715) was a German prince, son by his second marriage of Johann Ernst III, Duke of Saxe-Weimar. Despite his early death (he was just 18) he is remembered as a collector and commissioner of music and as a composer, some of whose concertos were arranged for harpsichord or organ by Johann Sebastian Bach, who was court organist in Weimar at the time. The Bach transcriptions were created roughly during the period July 1713–July 1714 between Johann Ernst’s return from Utrecht and the prince’s final departure from Weimar. There is some debate on Johann Ernst’s role in the creation of these arrangements, whether he commissioned some from one or both of the musicians or whether Bach, in particular, was studying some of the works collected by the prince for their own sake. There are suggestions that on a visit to Amsterdam in February 1713 the Prince may have heard the blind organist J. J. de Graff, who is known to have played keyboard arrangements of other composers’ concertos. In any case, Bach’s encounter with the prince’s collection, and especially the Italian music it contained, had a profound influence on the development of the composer’s musical style. (Source: Internet - wiki)