Easier Piano Pieces 65. Johann Hassler was a German pianist, organist and composer who travelled widely and spent the last 30 years of his life in Moscow, where most of his piano compositions were published. Op. 38 is an interesting pedagogic collection of 50 pieces which progresses around the circle of fifths through all the keys, starting with C major and A minor and working through the sharps and then the flats to end with F major and D minor.