A prepared piano is a piano that has had its sound altered by placing objects (preparations) 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. Many others have since written for the prepared piano.
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.
The phrase prepared piano is also sometimes applied to other kinds of preparations. Lou Harrison, for example, used something he called the tack piano, a piano with small nails stuck in the hammers to produce a more percussive sound. Conlon Nancarrow adapted his player pianos in a similar way, covering the hammers with metal and leather.
In pop music, German pianist Fritz Schulz-Reichel had considerable success on the hit parade with his prepared-piano recordings under the name Crazy Otto (Schräger Otto or "Slanted Otto" in German). During the craze, ragtime and blues pianist Johnny Maddox also recorded The Crazy Otto Medley with a tack piano. Both purportedly used thumbtacks in the felt hammers. More recently, the British electronic composer Aphex Twin used a prepared piano on his 2001 album Drukqs.
The most famous use of prepared piano in pop is Money by the Flying Lizards. Prepared with small pieces of tin and phone books, the minimalist song had a unique sound that turned it into a huge hit in the early 1980s. Today the song is something of a standard, used in many advertisements.