The piano is a musical instrument played by means of a keyboard. Widely used in Classical music for solo performances, ensemble use, chamber music, and accompaniment, the piano is also very popular as an aid to 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 felt-covered hammer to strike steel strings. The hammers rebound, allowing the strings to continue vibrating at their resonant frequency.[1] These vibrations are transmitted through a bridge to a sounding board that couples the acoustic energy to the air so that it can be heard as sound. When the key is released, a damper stops the string's vibration. See the article on Piano key frequencies for a picture of the piano keyboard and the location of middle-C. Pianos are percussion instruments. According to the Hornbostel-Sachs method of music classification, they are grouped with chordophones. The musical terms "piano" and "forte" are usually interpreted as "soft" and "loud", but this is not strictly what they mean in Italian. "piano" means here a plane or level, suggesting the normal level of playing. "Forte" would mean a stronger, more powerful level of playing, effectively louder than usual.