A player piano is a modified piano that "plays itself." The piano keys move according to a pattern of holes punched in an unwinding scroll. Unlike a music synthesizer, the instrument actually produces the sound itself, with the keys moving up and down, driving hammers that strike the strings. Like its counterpart, a player piano can be played by hand as well. When a roll is run through the instrument, the movement of its keys produce the illusion that an invisible performer is playing the instrument. Vonnegut uses the player piano as a metaphor to represent how even the most simple of activities, such as teaching oneself how to play the piano in one's spare time, has been replaced by machines instead of people. Early in the book, Paul Proteus's friend and future member of the Ghost Shirt Society, Ed Finnerty, is shown manually playing a player piano, suggesting the idea of humans reclaiming their animus from the machines. The book's most tragic character is Rudy Hertz, the machinist who was the prototype recorded by the machines. They are player pianos replicating his physical motions.
Vonnegut - Player Piano