Zhongyuan Yinyun

Zhongyuan Yinyun (simplified Chinese: ; traditional Chinese: ; pinyin: Zhōngyuán Yīnyùn), literally meaning "Rhymes of the central plain", is a rime book from the Yuan dynasty compiled by Zhou Deqing (周德清) in 1324. An important work for the study of historical Chinese phonology, it testifies many phonological changes from Middle Chinese to Old Mandarin, such as the reduction and disappearance of final stop consonants and the reorganization of the tones. Though often termed a "rime dictionary", the work does not provide meanings for its entries.

Zhongyuan Yinyun continued the tradition of Qieyun and other rime books. However, due to the phonological changes took place from the Sui dynasty to the Yuan dynasty, the information needed to be updated in accordance with the then phonological system.

From the middle of the 13th century to the end of the 14th century, Beiqu (北曲, Northern Verse) underwent quick development. The author of Sanqu, Zhou Deqing, delved into the research on Beiqu, discovering that it created many problems by not adhering to the rules of classical poetic composition. He thought that in order to better develop Beiqu, one would need to make a definite standard, especially in respect to language. According to his own experience, he was able to propose a set of rules for composing and reciting Běiqǔ, which came to be known as Zhongyuan Yinyun.

In the earlier rime books, characters are first grouped by tone, then by rime. However, in Zhongyuan Yinyun, the selected 5,866 characters, commonly rhymed in songs of the time, are first grouped into 19 rime groups, then further into four tonal groups: ping sheng yin (陰平 "feminine level tone"), ping sheng yang (陽平 "masculine level tone"), shang sheng (上聲 "rising tone"), qu sheng (去聲 "departing tone"). The traditional ru sheng (入聲 "entering tone") is assigned to three groups according to contemporary rules in some modern Ji-Lu Mandarin dialects. This novel way of dividing the traditional four tones is known as "dividing the level tones into yin and yang, assigning the entering tone to the other three tones" (平分陰陽，入派三聲).

...
Wikipedia