The major sources for the sounds of Old Chinese, covering most of the lexicon, are the sound system of Middle Chinese (7th century AD), the structure of Chinese characters, and the rhyming patterns of the Classic of Poetry (Shijing), dating from the early part of the 1st millennium BC.
The fact that the Qieyun system contains more distinctions than any single contemporary form of speech means that it retains additional information about the history of the language.