A prepared piano is a piano that has had its sound altered by placing objects between or on the strings.
The first composer to use it extensively was John Cage, who is often credited with inventing the instrument. Cage himself said he was greatly inspired by Henry Cowell’s experiments with the so called string piano, where the performer plucks and scrapes the strings of the piano directly.
In Cage’s use, the preparations are typically nuts, bolts and pieces of rubber to be lodged between and entwined around the strings. Some preparations make duller, more percussive sounds than usual, while others create sonorous bell-like tones. Additionally, the individual parts of a preparation like a nut loosely screwed onto a bolt will vibrate themselves, adding their own unique sound. By placing the preparation between two of the strings on a note which has three strings assigned to it, it is possible to change the timbre of that note by depressing the soft pedal on the piano (which moves the hammers so they strike only two strings instead of all three).
Although it is possible to prepare an upright piano in this way to some extent, it is far easier, and far more common, on a grand piano.
John Cage - A Room
Composed in 1943, originally conceived as the third part of She Is Asleep (see below). May be performed with or without preparations, which involve 11 notes. Most are to be prepared using bolts, one new material is a penny. The music is written down on a single staff and follows the structure 4, 7, 2, 5, 4, 7, 2, 3, 5 (numbers denote the number of bars dedicated to a particular part of the section), repeated twice.