Why Does a Harpsichord Have Two Manuals?

Two manuals do not give a harpsichord twice as many notes. Its strings are plucked, and pressing a key harder usually cannot vary the volume continuously as it can on a modern piano. A second manual lets the player move between preselected sets of strings and stops. On some instruments a coupler also lets one keyboard engage more than one set of jacks. What changes first is the combination and colour of the sound.

A 1744 harpsichord by Jacob Kirkman described by the Metropolitan Museum of Art has two manuals. The museum records several eight-foot and four-foot stops on the lower manual, further stops on the upper, and a coupler that can engage the upper manual from the lower. The eight-foot and four-foot labels indicate register: a four-foot set normally sounds an octave above an eight-foot set. The exact arrangement differs from instrument to instrument. The manuals therefore offer prepared choices of voice and colour; they are not two pianos stacked together.

When listening to a Baroque keyboard piece, notice how a change of manual can alter the brightness of an attack or the weight of a passage. A change of colour can clarify structure, but it cannot shape a phrase by itself. The player still has to judge note length, release and the distance between voices. For someone studying the modern piano, the point carries over: musical layers are made not only by playing louder or softer but also by deciding which sounds belong together.

Sources:
https://www.metmuseum.org/art/collection/search/504610
https://www.metmuseum.org/art/collection/search/504339