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Extravagant praise for the clavichord, like this passage by the German poet and musician C.F. Daniel Schubart, was common in the eighteenth-century. The clavichord reached its apogee in terms of repertoire and popularity in Germany in the mid- to late-18th century with composers such as Carl Philipp Emanuel Bach and the 'Bachists' who succeeded him, but was hardly a new instrument. It had already been in existence for several hundred years, and was the only touch-sensitive keyboard instrument until the invention of the Florentine piano in the late seventeenth-century. Like the pianos of Bartolomeo Cristofori, the clavichord allowed the musician to vary the loudness and softness of the sound through finger pressure alone. Its quiet, extraordinarily varied tone was thought to resemble the human voice, making the instrument the 'perfect mirror for the finer shades of feeling', 'the thrilling confidant of solitude'.
The clavichord is usually built in a rectangular case. Its strings stretch from left to right across the case and pass over a bridge or several bridges glued to the soundboard, which occupies the right side of the instrument. The simple action is generally found on the left and in the center of the instrument, and includes the keyboard and keylevers which extend from the keys to the back of the case. A brass blade called a tangent is driven into the back of each keylever. When the player's finger depresses the key, the far end of the keylever rises, causing the tangent to strike a string or pair of strings which then vibrate. The clavichord, like the piano, is therefore a percussion instrument, and differs from the harpsichord, whose strings are plucked. The tangent performs two functions as it strikes the strings: it not only makes them vibrate but also determines their vibrating length "in much the same way a guitarist determines the pitch that a string will sound by pressing against one of the frets on the fingerboard of his instrument. Accordingly, by positioning a series of tangents so they will strike the same pair of strings at different points along their length, a series of different notes can be sounded." Earlier instruments in which one pair of strings produced several notes were known as fretted clavichords. Unfretted clavichords, in which each pair of strings produced a single note, were more common from the late seventeenth-century on. The fact that the tangent is in direct contact with the strings for the duration of the note means that the player can vary the pitch of that note by increasing and decreasing the pressure on the key. This produces a "subtle pulsating prolongation" of the tone known as Bebung, which literally means "quaking". The ability to produce this vibrato is one of the "most celebrated characteristics" of the clavichord. Few indications for Bebung appear in musical scores, however, and it is a special effect that should be used sparingly.
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