During his years at Weimar Bach made a number of keyboard arrangements of concertos and instrumental movements by other composers. His arrangements of concertos by Vivaldi, six of them for harpsichord and three for organ, remind us of the strong influence Vivaldi exercised over Bach's Instrumental compositions. The sixteen arrangements for harpsichord include a keyboard version of an oboe concerto by Alessandro Marcello, a violin concerto by Telemann and three concertos by Prince Johann Ernst of Saxe-Weimar. The six concertos transcribed for organ also include arrangements of two concertos by Duke Johann Ernst. The latter was a nephew of Bach's employer and a pupil for keyboard and for composition of Johann Gottfried Walther, organist of the Weimar Stadtkirche. His principal instrument was the violin and Telemann wrote for him a set of six sonatas for violin and clavier. Johann Ernst died in 1715 at the age of nineteen, leaving nineteen instrumental works. Of these six concertos were published posthumously by Telemann in 1718.