 A similar structure can also be seen in the violin concerti of, for example, Vivaldi, who established the form, along with the three-movement concerto structure, and Viotti, wherein the concerto is divided into six sections.The keyboard parts of the concertos were almost invariably based on material presented in the ritornelli, and it was probably J.C. Bach, whom Mozart admired, who introduced the structural innovation of allowing the keyboard to introduce new thematic material in its first entry.Concertos Nos 1–4 (K. 37, 39, 40 and 41) are orchestral and keyboard arrangements of sonata movements by other composers.