The piano is a musical instrument played by means of a keyboard. Widely used in Western music for solo performance, 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 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. 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. Pianos are sometimes classified as both a percussion and a stringed instrument. According to the Hornbostel-Sachs method of music classification, it is grouped with Chordophones.

The word piano is a shortened form of the word pianoforte, which is seldom used except in formal language and derived from the original Italian name for the instrument, clavicembalo gravicembalo col piano e forte (literally harpsichord with soft and loud). This refers to the instrument's responsiveness to keyboard touch, which allows the pianist to produce notes at different dynamic levels by controlling the speed with which the hammers hit the strings.

Early history

The piano is founded on earlier technological innovations. The first string instruments with struck strings were the hammered dulcimers originating from the Persian traditional musical instrument santur. During the Middle Ages, there were several attempts at creating stringed keyboard instruments with struck strings, the earliest being the hurdy gurdy which has uncertain origins. By the 17th century, the mechanisms of keyboard instruments such as the clavichord and the harpsichord were well known. In a clavichord the strings are struck by tangents, while in a harpsichord they are plucked by quills. Centuries of work on the mechanism of the harpsichord in particular had shown the most effective ways to construct the case, soundboard, bridge, and keyboard.

Cristofori's great success was in solving, without any prior example, the fundamental mechanical problem of piano design: the hammer must strike the string, but not remain in contact with it (as a tangent remains in contact with a clavichord string) because this would damp the sound. Moreover, the hammer must return to its rest position without bouncing violently, and it must be possible to repeat a note rapidly. Cristofori's piano action served as a model for the many different approaches to piano actions that followed. While Cristofori's early instruments were made with thin strings and were much quieter than the modern piano, compared to the clavichord (the only previous keyboard instrument capable of minutely controlled dynamic nuance through the keyboard) they were considerably louder and had more sustaining power.