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A key is a specific part of a musical instrument. The purpose and function of the part in question depends on the instrument.
A key is typically round or roundish in shape, or square or rectangular in the case of some instruments. It is usually pressed down in order to play a certain musical note, and can come in varying shapes, sizes, and colors, even on the same instrument. For example, piano keys are either white or black, with the black keys being smaller and thinner, and the white keys being larger and thicker. Keys may also serve different functions on the same instrument.
Scooby-Doo, Where Are You!
Insert details here. (A Gaggle of Galloping Ghosts)
Velma Dinkley played the keys of the notes of F-E-E-D on an organ in consecutive order to allow the gang to escape a room where the walls were closing in on them. The notes opened up a trap door in the floor, which the gang proceeded down into in order to escape. (A Night of Fright is No Delight)