So

The so is also held vertically and produces sixteen notes by the sixteen different bamboo pipes which are put into a wooden frame. The length of the bamboo pipe becomes shorter when it comes into the middle from the both edges. However, the length of the pipes outside is arranged by regular length in order to play with ease. Its bamboo pipe is made of a firm haejuk.

Players hold both sides of the frame with their both hands and then they blow the blowpipe with a lower lip as the tanso is played. But it is not po;)bible to produce a sustained or high-pitched sound. The so is found in the tomb painting of the old fresco in Koguryo, but it has been used since the Koryo dynasty and currently is using in Munmyo-cheryeak.

This instrument is similar to the wings of the Chinese phoenix in shape, and is called bongso or Chinese phoenix, 16 bamboo pipes are inserted into the wood frame, and the end of the pipes is stopped with wax.