The piano is among the foremost instruments used in classical music. It produces sound by striking metal strings with felt covered hammers. At first glance this system seems to be well described by the laws of transversely vibrating strings, but there are many subtle effects that change the sound produced from what this simple model predicts. Of these effects, the most important is the nonlinear interaction between the hammer and the string. These complications seem to account for a large part of the charm of the piano sound.