How are characters defined in musical terms?

Keller's room contains two pianos: a Bosendorfer grand and a peeling Wertheim upright. Keller always sits at the grand and has Paul sit at the old upright. The different pianos symbolize the gap between their musical skills. Paul continues to sit at the upright throughout his lessons, symbolizing that while he is becoming a better pianist he is still never as good as Keller is. Paul's desire to play at the grand parallels his desire to know more about Keller's past. When he does play the grand when Keller is not there, Paul notices the pictures of Keller's wife and son, gaining an insight into the life of his teacher. Keller's piano also symbolizes how for him music has become a refuge from the tragedies of the world. This is shown when Keller hides under his piano during the cyclone, and the piano shields him from the disaster.