The piano is a musical instrument played using a keyboard. It is widely employed in classical, jazz, traditional and popular music for solo and ensemble performances, accompaniment, and for composing and rehearsal. Although the piano is not portable and often expensive, its versatility and ubiquity have made it one of the world’s most familiar musical instruments. An acoustic piano usually has a protective wooden case surrounding the soundboard and metal strings, and a row of 88 black and white keys (52 white, 36 black). The strings are sounded when the keys are pressed, and silenced when the keys are released. The note can be sustained, even when the keys are released, by the use of pedals. Pressing a key on the piano’s keyboard causes a padded hammer to strike strings. The hammer rebounds, and the strings continue to vibrate at their resonant frequency. These vibrations are transmitted through a bridge to a soundboard that amplifies by more efficiently coupling the acoustic energy to the air. When the key is released, a damper stops the strings' vibration, ending the sound. Although an acoustic piano has strings, it is usually classified as a percussion instrument because the strings are struck rather than plucked.