The ruan (阮) or moon guitar is a four-stringed Chinese instrument similar to the pipa. It is also sometimes called the qin pipa (秦琵琶) and ruanxian (阮咸).
The most common ruan is the zhongruan (中阮) or tenor ruan. The bass ruan (大阮) is also fairly common; less frequently heard are the soprano (高音阮), alto (小阮) and contrabass: Diyinruan (低音阮) versions.
In this video, Wang Yiping (王一平) plays “Yi Dance”, a song adapted from folk melodies of the Yi minority. if you can’t see the video player below, you can also watch it on Youtube and Tudou.
This video was shot and edited by Jonah Kessel.
More about the ruan and “Yi Dance”
Wikipedia: Ruan
Philmitic: Yi Dance (played on pipa)
Paul Knoll: Ruan