Although Old Chinese is known from written records beginning around 1200 BC, the logographic script provides much more indirect and partial information about the pronunciation of the language than alphabetic systems used elsewhere. Several authors have produced reconstructions of Old Chinese phonology, beginning with the Swedish sinologist Bernhard Karlgren in the 1940s and continuing to the present day. The method introduced by Karlgren is unique, comparing categories implied by ancient rhyming practice and the structure of Chinese characters with descriptions in medieval rhyme dictionaries, though more recent approaches have also incorporated other kinds of evidence.
Although the various notations appear to be very different, they correspond with each other on most points. By the 1970s, it was generally agreed that Old Chinese had fewer points of articulation than Middle Chinese, a set of voiceless sonorants, and labiovelar and labio-laryngeal initials. Since the 1990s, most authors have agreed on a six-vowel system and a re-organized system of liquids. Earlier systems proposed voiced final stops to account for contacts between stop-final syllables and other tones, but many investigators now believe that Old Chinese lacked tonal distinctions, with Middle Chinese tones derived from consonant clusters at the end of the syllable.