Middle Chinese or the Qieyun system (QYS) is the historical variety of Chinese recorded in the Qieyun, a rime dictionary first published in 601 and followed by several revised and expanded editions. The Swedish linguist Bernhard Karlgren believed that the dictionary recorded a speech standard of the capital Chang'an of the Sui and Tang dynasties. However, based on the preface of the Qieyun, most scholars now believe that it records a compromise between northern and southern reading and poetic traditions from the late Northern and Southern dynasties period. This composite system contains important information for the reconstruction of the preceding system of Old Chinese phonology.

A rime dictionary, rhyme dictionary, or rime book is an ancient type of Chinese dictionary that collates characters by tone and rhyme, instead of by radical. The most important rime dictionary tradition began with the Qieyun (601), which codified correct pronunciations for reading the classics and writing poetry by combining the reading traditions of north and south China. This work became very popular during the Tang dynasty, and went through a series of revisions and expansions, of which the most famous is the Guangyun (1007–1008).