南朝梁陳以後的文人，在韻文創作中區別平聲和上去入三聲，這兩類的對立，後來發展成了「平仄（側）」的概念，但其原來的性質已經不很清楚。現在我們能依靠日本漢字音等資料考定盛中唐時期各聲調的擬音，它代表盛中唐時期的平仄的實質，其特徵可以概括為「平低仄高」。如此構擬，我們還能解釋現代北方方言古全濁聲母清化時為什麼逢平讀送氣、逢仄讀不送氣的問題。四聲和平仄在晚唐五代至北宋之間基本消逝，主要原因是「陰陽調」的分化和發展。

　　In the Late Southern Dynasties (6th century AD), for rhetorical purposes, Chinese literati induced the four tonal categories (si-sheng) into two groups: the level tone and the other three tones (rising, departing, entering). These two groups subsequently developed into the so-called level-oblique distinction, and the original tonal feature of this distinction was not investigated enough. In the late 20th century some scholars succeeded in the reconstruction of tonal values of the Middle Tang period, and presumed that the feature of level-oblique distinction of Middle Tang was: [level-low, oblique-high]. Such reconstruction of the tones can explain why the Northern Chinese dialects (Mandarin dialects) in the process of devoicing, developed into voiceless aspirates in the level tone, voiceless non-aspirates in the oblique tones.
　　Both of the four-tone system and the level-oblique distinction disappeared during the 8-11th centuries AD, and the most important cause of this development was the evolution of Yin-Yang registers.

平仄; 漢語; 陰陽調; 濁音清化; 韻律

level-oblique; Chinese; Yin-Yang registers; devoicing prosody