One of the numerous instruments believed to be indigenous to China, emerged during the Zhou Dynasty (c. 11th to 3rd centuries BCE). By the 8th century CE, it had 12 or 13 strings of silk, increasing to 16 (ultimately of copper or steel wire) and, from the mid-20th century, to 18 or 21. It's a popular multi-stringed zither, with pitch-defining movable bridge under each of its strings. The strings are tuned to a pentatonic scale without half-steps (usually so, la, do, re, mi) over a 3-plus octave range.