Klas Bernhard Johannes Karlgren was a Swedish sinologist and linguist who pioneered the study of Chinese historical phonology using modern comparative methods. In the early 20th century, Karlgren conducted large surveys of the varieties of Chinese and studied historical information on rhyming in ancient Chinese poetry, then used them to create the first ever complete reconstructions of what are now called Middle Chinese and Old Chinese.

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.