The clavichord is the direct ancestor of the piano.
The keys raise so-called tangents (small rectangular pieces of brass) which hit the strings but remain in contact with them, simultaneously producing a damping effect.
The big advantage of the clavichord is that loudness can be continually varied, and the touch is extremely sensitive: it gives the illusions of tapping the strings with one’s own fingertips.
Unfortunately the damping effect of tangent-on-string reduces the over-all volume of the sound that the clavichord serves primarily for practice, or for solo playing in small and particularly resonant rooms.
The clavichord may be fretted, with one string used to produce two or three notes a semitone apart, or unfretted, with one string per note.
J. S. Bach – Goldberg, Aria