The traditional koto has 13 strings. Moveable ivory bridges, called ji, are placed under each string, and moving them up or down the length of the koto raises or lowers the pitch. There are about 15 set positions for the ji, known as choshi, and they determine the overall tuning of the instrument. All traditional koto music uses one or more of these choshi.
The number of strings has remained constant since the koto was imported from China as part of the gagaku Imperial court orchestra during the Nara Period (eighth century). In the 20th century, however, hogaku musicians began experimenting with instrumental innovations and modern playing techniques.
One obvious innovation was to add more strings to the koto in search of a greater pitch range and more flexibility in the tuning.