The piano is a musical instrument played by means of a keyboard. It is one of the most popular instruments in the world. Widely used in classical and jazz music for solo performances, ensemble use, chamber music and accompaniment, the piano is also popular as a tool for composing and rehearsal. Although not portable and often expensive, the piano's versatility and ubiquity have made it one of the world's most familiar musical instruments.
Pressing a key on the piano's keyboard causes a padded (often with felt) hammer to strike steelstrings. The hammers rebound, and the strings to continue to vibrate at their resonant frequency. These vibrations are transmitted through a bridge to a sounding board that more efficiently couples the acoustic energy to the air. The sound would otherwise be no louder than that directly produced by the strings. When the key is released, a damper stops the string's vibration and the sound. See the article on Piano key frequencies for a picture of the piano keyboard and the location of middle-C. In the Hornbostel-Sachs system of instrument classification, pianos are considered chordophones.