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.

Old Chinese, also called Archaic Chinese in older works, is the oldest attested stage of Chinese, and the ancestor of all modern varieties of Chinese. The earliest examples of Chinese are divinatory inscriptions on oracle bones from around 1250 BC, in the Late Shang period. Bronze inscriptions became plentiful during the following Zhou dynasty. The latter part of the Zhou period saw a flowering of literature, including classical works such as the Analects, the Mencius, and the Zuo zhuan. These works served as models for Literary Chinese, which remained the written standard until the early twentieth century, thus preserving the vocabulary and grammar of late Old Chinese.