Clavichords

Clavichords are made in two fashions: fretted or unfretted. In the former, several adjacent keys actually play on the same unison pair of strings by contacting them in different places, effectively changing the sounding length. The same process works on guitars, lutes, and other fretted instruments, hence the application of the name to the keyboard instrument. In the bass the distance that would be necessary between the striking points if the notes were fretted becomes impractical. The lowest notes therefore have their own unison pairs of strings. The sharing of strings for adjacent notes in the middle and treble ranges results in a reduction of the total number of strings necessary, lowering the amount of tension on the case and soundboard, and making tuning a relatively easy matter.

Earlier fretted instruments usually had three notes per pair of strings, and are known as triple-fretted. An alternate arrangement, where only chromatic pairs of notes like c and c# share the same strings co-existed, and this system became preferred during the 18th century. These double-fretted instruments have fewer conflicts occurring from the need to play two adjacent notes simultaneously, and they allow a greater variety of articulation than is possible with the triple-fretted instruments.

In order to provide total freedom of articulation, makers eventually turned during the 18th century to unfretted instruments where every key has its own pair of strings. Instruments of this type are essential to the idiomatic clavichord repertoire of composers like C. P. E. Bach. These five-octave instruments can be quite large, and are no longer easily portable like the fretted examples. They feature a sophisticated, velvety sound, but because of the increased pressure on the bridge from the greater number of strings, they often have less presence than their unfretted relatives.

The double-fretted clavichord was never entirely replaced by the unfretted type, however, and they continued to be made throughout the 18^th century. A number of fine larger examples with compasses of four and a half or five octaves can be found, and many clavichord aficionados regard these as the ultimate instruments. Even larger fretted instruments with six octaves were made, probably as inexpensive substitutes for the square piano during the early 19th century.

source: http://www.mapleharpsichords.com/