According to the Samguksagi a history of the Three Kingdoms of Korea,
the gayageum was developed around the 6th century in the Gaya
confederacy by King Gasil (also known as Haji of Daegaya) after he
observed an old Chinese instrument. He ordered a musician named Wu Ruk
to compose music that could be played on the instrument. The original name
was gayago and later gayageum. The gayageum was then further improved
by Wu Ruk during the reign of Jinheung in the Shilla Dynasty.
The ancient gayageum of King Gashil was called by several names,
including beopgeum (law-zither, ??), pungnyu (elegance, ??), or jeongak (right music, ??) gayageum. It is normally associated with
accompaniment for court music, chamber music, and lyric songs. This type
of gayageum has a wider spacing between the strings and plays slower
tempo music such as Yeongsan-hoesang and Mit-doduri.
The sanjo gayageum is believed to have evolved in the 19th century with the
emergence of sanjo music—which literally means scattered melodies, a
musical form that involves fast tempos and some improvisation. The sanjo
gayageum version of the instrument has closer string spacing and a shorter
length to let musicians play the faster passages required for sanjo (Choi
2005). The sanjo gayageum is now the most wide spread form
of gayageum.(KCMPC 2001). All traditional gayageum use silk strings,
though, since the late 20th century, some musicians use nylon.
Modern versions of the gayageum with more strings often use nylonwrapped steel strings, similar to those used for the Chinese guzheng (Choi
2005). Brass strings have also been introduced to produce a louder sound,
which is preferred for accompanying dance (Park 2004). To play modern
music, gayageum with a greater number of strings have been developed,
increasing the instrument's range. Gayageum are available with 13, 17, 18,
21, 22, or 25 strings (Choi 2005), though instruments with more strings are
available custom-made. The 21 string gayageum is normally found in North
Korea.