The array of keys a body speaks to a machine through. For many, the most-used physical interface there is.
Old English caeg (key) joined to bord (a plank, board); a 19th-century compound first naming the piano's array of keys, then the typewriter's, then the array a body speaks to a machine through.
For most bodies, the single most-used physical surface between a person and a machine — an array spoken to rather than merely pressed.