At the end of 1929, Respighi had conductor Serge Koussevitzky forward a proposal to Sergei Rachmaninoff which involved permission to orchestrate a selection of pieces from his two "Études-Tableaux" sets for piano. An enthusiastic Rachmaninoff accepted the offer and supplied Respighi with the program descriptions behind five pieces which were previously kept secret. Koussevitzky conducted Respighi's orchestrations, entitled "Cinq Études-Tableaux", for the premiere with the Boston Symphony Orchestra in November 1931. He wrote that Respighi's arrangements were "very good" and demanding of the orchestra, which required eight rehearsals. Rachmaninoff thanked Respighi for his work and in particular, for being faithful to the original scores. Later in 1930, Respighi completed a commission piece to commemorate the fiftieth anniversary of the Boston Symphony Orchestra. The result was "Metamorphoseon, Modi XII", an orchestral piece containing a theme and eight variations.