A key is a specific part of a musical instrument. The purpose and function of the part in question depends on the instrument.
Key Instruments are
The piano may be the best known and loved of all musical instruments. It also has the broadest range of any instrument, so music for all other instruments can be composed on it. It can be played solo, but most other instruments, including the voice, use the piano for accompaniment. Technically, the piano may also be the most complicated musical instrument with over 2,500 parts.
The piano is a stringed instrument. Its many parts are organized into five general structural and mechanical areas of either grand or vertical pianos. These are: the case of the wing-shaped grand piano (or the cabinet of the vertical or upright piano); the soundboard and the ribs and bridges that are its components; the cast iron plate; the strings; and, collectively, the keys, hammers, and piano action or mechanism. The case has many structural parts for attaching legs and tuning pins, but perhaps the rim and the keybed or shelf where the keys and piano action will be installed are most important. The soundboard amplifies the vibrations of the strings, which are transmitted through bridges.
The cast iron plate is installed over the soundboard and pinblock (part of the case), and it provides the strength to anchor the strings under tension. Nose bolts and perimeter bolts anchor the plate to the braces and inner rim of the case. The 220 to 240 strings of the piano are attached to hitch pins along the curved edge of the cast iron plate and to tuning pins across the front of the piano, roughly parallel to the keyboard. The piano action is still more complicated and includes the keys, hammers, and mechanism or action