Derived mostly from pansori singing and originally largely improvised, "sanjo" became (especially in the stylistic school of Master Choi Ok-Sam) a structured instrumental suite: On the 12 silk strings of the "gayageum" zither, Kim Hae-Sook paints with virtuosic serenity, a musical landscape of exaltation and restrained emotion. The gayageum has a long oblong body and rests horizontally on the lap when played. The sanjo gayageum has 12 strings that symbolise the 12 months of the year, which are plucked with bare fingers of the right hand and then, while the sound lingers (not more than three seconds), either pressed, slid down or vibrated with the left hand. Gayageum sanjo involves a variety of out-of-tune tones, a technique commonly known as nonghyeon. This technique is widely used for musical expression in traditional Korean music.