A prepared piano is a piano that has had its sounds temporarily altered by placing bolts, screws, mutes, rubber erasers, and/or other objects on or between the strings. Its invention is usually traced to John Cage's dance music for Bacchanale (c. 1938), created without room for a percussion orchestra. Cage has cited Henry Cowell as an inspiration for developing piano extended techniques, involving strings within a piano being manipulated instead of the keyboard. Typical of Cage's practice as summed up in the Sonatas and Interludes (1946–48) is that each key of the piano has its own characteristic timbre, and that the original pitch of the string will not necessarily be recognizable. Further variety is available with use of the una corda pedal.
Ferrante & Teicher between 1950 and 1980 used partially prepared pianos for some of their tunes in their albums. Other musicians, such as Denman Maroney use prepared piano for performances, whereas Cor Fuhler and Roger Miller have developed their own ways of using prepared piano in their musical albums. Additionally, notable contributors to the subsequent repertoire include Lou Harrison, Pauline Oliveros, James Tenney, and Christian Wolff.
When a properly prepared piano has been "unprepared", it should be impossible for anyone to tell that it had ever been prepared. Changes causing less easily reversible damage can be served by permanently dedicating an instrument, such as the tack piano. Other techniques related to prepared piano include the Acoustisizer.
Cage frequently cited Henry Cowell (1897–1965) as the primary inspiration for the prepared piano. Cowell pioneered piano extended techniques for what he dubbed "string piano", involving reaching inside the piano and pluck, sweep, scrape, thump, and otherwise manipulate the strings directly, rather than using the keyboard. He developed these techniques in numerous pieces such as Aeolian Harp (1923) and The Banshee (1925). Pieces of paper were called for in several early 20c works, the buzzing effect reminiscent of the parchment 'bassoon' pedal of early fortepianos. In his Ragamalika (1912–22), based on the classical music of India, French composer Maurice Delage (1879–1961) calls for a piece of cardboard to be placed under the B♭ in the second line of the bass clef to dampen the sound, imitating the sound of an Indian drum.
In his Chôros No. 8, a 1925 work for large orchestra, Heitor Villa-Lobos instructs the 2nd pianist to insert pieces of paper between the strings  Maurice Ravel's L'enfant et les sortilèges (1920-1925) calls for Luthéal, but allows piano with paper to substitute.
See also: Works for prepared piano by John Cage
The invention of the "prepared piano", per se, is usually traced to John Cage. Cage first prepared a piano when he was commissioned to write music for Bacchanale, a dance by Syvilla Fort in 1938. For some time previously, Cage had been writing exclusively for a percussion ensemble, but the hall where Fort’s dance was to be staged had no room for a percussion group. The only instrument available was a single grand piano. After some consideration, Cage said that he realized it was possible "to place in the hands of a single pianist the equivalent of an entire percussion orchestra ... With just one musician, you can really do an unlimited number of things on the inside of the piano if you have at your disposal an exploded keyboard".
Main article: Tack piano
Strictly speaking, a tack piano is not a prepared piano, since
Although the tacks can be removed from the hammers, inserting them causes permanent damage to the felt; for this and other reasons, the use of tacks is generally discouraged by piano technicians.
The Acoustisizer is an electroacoustic musical instrument built from a small grand piano with built-in speakers, magnetic guitar pickups, PZMs, and prepared piano strings. It was built as part of a graduate thesis project at California State University Dominguez Hills by Bob Fenger (1983), a student of Richard Bunger (author of the Well Prepared Piano). Speakers are built into the bottom of the instrument, redirecting its own amplified sound back onto the sounding board, with strings and magnetic pickups creating an amplitude intensity loop, which in turn drives and vibrates suspended kinetic oscillators (assemblages of vibration sensitive materials). Secondary control parameters allow extraction of vibration and sound phenomena from the kinetic oscillators through a series of proximity microphones and PZMs (piezo-electric contact mics). An article by the inventor was published in Experimental Musical Instruments Magazine April 1991, Nicasio, California. It includes pictures of the kinetic oscillators and stages of the construction process, including an underbody view of the speaker system configuration.
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In 'Little Fishes,' Eno plays prepared piano and Farfisa organ.