A prepared piano is a piano that has had its sound altered by placing objects (preparations) between or on the strings or on the hammers or dampers.
The phrase prepared piano is also sometimes applied to other kinds of preparations. The tack piano is a piano that has been altered by inserting thumbtacks or small nails into the striking end of each hammer, so that the instrument will produce a more percussive sound and brighter timbre. The resulting tone often resembles the sound of a very old and derelict piano whose hammers have been hardened and compressed over decades of use. The tack piano has been used primarily in honky-tonk-style piano playing, or to make a piano sound like an antique piano that might have been heard in a saloon or brothel around the early 20th century.
The application of tacks is generally discouraged by piano technicians as the tacks can drop off the hammers and lodge in the strings or jam the mechanism, or the fact that placing tacks inside felt hammers permanently damages the felt, which is stretched at high tension over the hammer heads, rendering it unvoicable and likely to split.
Villa-Lobos's Choros no. 8
In his 1925 work for two pianos and large orchestra, Heitor Villa-Lobos added to his score instructions to the pianist to insert pieces of paper between the strings and the hammers to attain a certain sonority.
Another precedent to the prepared piano was an experiment by the French composer Maurice Delage (1879–1961): his Ragamalika (1912–22), based on the classical music of India, calls for a piece of cardboard to be placed under the B-flat in the second line of the bass clef to dampen the sound, imitating the sound of an Indian drum.
The Acoustisizer is an electro acoustic musical instrument built from a small grand piano with built in speakers, magnetic guitar pickups, PZMs, and prepared piano strings. Built as part of a graduate project thesis 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, 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 phenomenon from the kinetic oscillators through a series of proximity microphones and PZMS (piezo electric contact mics). An article was written by the inventor and was published in Experimental Musical Instruments Magazine April 1991, Nicasio CA It includes pictures of the kinetic oscillators and stages of the construction process including an underbody view of the speaker system configuration.  See also Public Prepared Piano at Street piano 3.0 (Public Piano Project in Joshua Tree CA).  
John Cage and later composers
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”.
In popular music
- "All Tomorrow's Parties" from The Velvet Underground & Nico (1967), John Cale prepared his piano with a chain of paper clips.
- Denman Maroney performs on what he has dubbed hyperpiano, which "involves stopping, sliding, bowing, plucking, striking and strumming the strings with copper bars, aluminum bowls, rubber blocks, plastic boxes and other household objects."
- Since 1982, Roger Miller has fashioned his own take on prepared piano in his work. Initially featured on Mission of Burma's 45 "Trem II", Miller has since released many albums (Ace of Hearts, SST, New Alliance, Atavistic, Matador) incorporating this technique. His concert hall compositions, from 2009 to present, often utilize prepared piano."
- Anna Stella Schic (1987). Villa-Lobos, Souvenirs de l'Indien Blanc, Actes Sud, p. 82.
- Pasler, Jann (2000). "Race, Orientalism, and Distinction in the Wake of the 'Yellow Peril'." In Western Music and Its Others: Difference, Representation, and Appropriation in Music, ed. Georgina Born and David Hesmondhalgh. Berkeley, Los Angeles, and London: University of California Press, p. 107.
- Fenger, Bob Icon. "The Acoustisizer"  Nicasio CA, Volume 6 #6 April 1991.
- Fenger,Bob, "The Acoustisizer"  Joshua Tree CA, April 1991
- Connell, Rich. "Inventor Creates New Sound of Music" Los Angeles Times Los Angeles, 21 November 1982.
- Bronson, James. "Acoustisizer creator's world is music"  Palos Verdes CA, 2 December 1982.
- Woods, Randy. "New Instrument Offers Sounds of Star Wars"  Los Angeles, 2 December 1982.
- Takahama, Valerie. "High-tech piano creates a new sound" Long Beach Press TelegramLong Beach, 2 December 1982.
- Bronson, James. "Acoustisizer creator is scaling new heights" Daily BreezeSouth Bay LA, 17 December 1982.
- Staff writer. "But Does He Burn It After Each Performance?" Keyboard Magazine Cupertino CA, February 1983.
- Fenger, Bob. "The Acoustisizer" Experimental Musical Instruments Nicassio CA, Volume VI #6 April 1991.
- Gast, Dena M. "Music sends environmental message"  Joshua Tree CA, 26 May 1995
- Cage, John, and Daniel Charles (1981). For The Birds: John Cage in Conversation with Daniel Charles. Marion Boyers London. ISBN 0-7145-2690-8.
- Mitchell, Tim Sedition and Alchemy : A Biography of John Cale, 2003, ISBN 0-7206-1132-6
- "Philadelphia FRINGE Festival 2000 - Hyperpiano". Archived from the original on 2007-12-31. Retrieved 2005-12-23..
- Pareles, Jon (1987-06-05). "Music: Roger Miller". NYTimes.com (The New York Times). Retrieved 2014-02-15.
- Burke, Kathleen (2012-06-19). "Fuse Feature: M2 — 'At Land’s Edge' Album Review/Interview". ArtsFuse.org. The Arts Fuse. Retrieved 2014-02-15.
- Gottschalk, Jennie (2013-07-02). "The Mush Race of Boston: The SICPP 2013 Iditarod". NewMusicBox.org. New Music USA. Retrieved 2014-02-15.
- Bunger, Richard (1973). The Well-Prepared Piano. Colorado Springs: Colorado College Music Press
- Fürst-Heidtmann, Monika (1979). Das präparierte Klavier des John Cage. Gustav Bose Verlag Regensburg. ISBN 978-3-7649-2183-5.
- Dianova, Tzenka (2008). John Cage's Prepared Piano: The Nuts & Bolts. Mutasis Books Victoria. ISBN 978-0-9809657-0-4.
- 'Are You Prepared' 17-key Online Prepared Piano by Andreas Busk. Site includes free downloadable prepared piano sampler for Ableton Live, NI's Kontakt and Logic's EXS Sampler
- The Sound Collector - The Prepared Piano of John Cage by Tim Ovens (PDF version).
- If you build it, they will come! essay by Kyle Gann, includes video performance of preparation by Margaret Leng Tan (here).
- Prepared Piano Sample Set - By Tom Gersic. Some free, others cheap
- Prepared Piano Max/MSP-Object - By Dr. Stefan Bilbao, ported to Max/MSP by Thomas Resch
- Jingle Bells on Prepared Piano - By George Aroshidze
- Prepared piano demonstration and performance by Richard Bunger
- Epitonic.com: John Cage performed by Margaret Leng Tan, featuring In the Name of the Holocaust