William H. Baxter's transcription for Middle Chinese is an alphabetic notation recording phonological information from medieval sources, rather than a reconstruction.
The centre of the study of Chinese historical phonology is the Qieyun, a rime dictionary created by Lu Fayan in 601 CE as a guide to the proper reading of classic texts.
[1][2] A series of rime tables from the Song dynasty applied a sophisticated analysis to the Qieyun system, though the language had changed in the interim.
[3][4] The division III finals can be further subdivided on the basis of their distribution: There have been many attempts to reconstruct the sounds or phonemes of the Qieyun system, conventionally called Early Middle Chinese, yielding a series of alphabetic transcriptions.
The custom in Chinese scholarship is to neutrally describe a syllable with a string of six characters identifying its 攝 shè, whether it is 開 kāi or 合 hé, the division, tone, Guangyun rime and initial.