Klas Bernhard Johannes Karlgren (15 October 1889 – 20 October 1978) was a Swedish sinologist and linguist who pioneered the study of Chinese historical phonology using modern comparative methods. In the early 1900s, Karlgren conducted large surveys of a number of Chinese dialects and studied historical information on rhyming in ancient Chinese poetry, then used them to create the first ever complete reconstructions of what is now called Middle Chinese and Old Chinese.

Bernhard Karlgren was born on 15 October 1889 in Jönköping, Sweden. His father, Johannes Karlgren, taught Latin, Greek, and Swedish at the local high school. Karlgren showed ability in linguistics from a young age, and was interested in Sweden's dialects and traditional folk stories. He mastered classical languages and was an accomplished translator of Greek poetry into his native language. He displayed an early interest in China, and wrote a drama, The White Hind, set in that country in his early teens. His first scholarly article, a phonetic transcription, based on a system devised by Johan August Lundell, of traditional folk stories from his native province of Småland, was completed when he was 14. and published in 1908 when he was only 18 years old. He studied Russian at Uppsala University under Johan August Lundell, a Slavicist interested in comparative linguistics. He graduated in 1909 with a bachelor's degree in Nordic, Greek, and Slavonic languages. Although he initially intended to specialize in the Scandinavian languages, on the advice of his elder brother Anton Karlgren (1882–1973) he decided to focus on Chinese instead, attracted to it also by the fact that, as Lundell had told him, Chinese contained a great number of dialects. He departed for St. Petersburg, where, under the guidance of Vasily Vasilyev, the city had created one of the major European centres for the study of Chinese. While there, Karlgren, studying under A. I. Ivanov, won a grant to study Chinese dialects, even though he had no background in Chinese at that point.

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