 It also cites some ancient books that are no longer extant, and are only known through "Jingdian Shiwen".Bernhard Karlgren considered "Jingdian Shiwen" and the 601 "Qieyun" rime dictionary as the two primary sources for reconstructing Middle Chinese.Many studies in Chinese historical linguistics (for instance, see References) use the important "Jingdian Shiwen" data.