Which of his pupils did Haydn have in mind when he composed his humorous “Divertimento à due per il Clavicembalo solo”? It was written at a time when works for piano four-hands were still the exception – and instead of “Primo” and “Secondo”, the oldest copies use the terms “La parte del Maestro” and “dello Scolare”. Haydn’s wit keeps surfacing; for instance, in the idea that the teacher plays first, then the pupil, and in the last bars of the variation movement the hands of the players cross ove...