The book that documents Mandarin's birth
In 1324, in the middle decades of the Yuan dynasty, a playwright and music theorist named Zhou Deqing (周德清) published Zhōngyuán Yīnyùn (中原音韻, 'Sounds and Rhymes of the Central Plain'). Its stated purpose was practical: help composers of 雜劇 (zájù, the wildly popular Yuan opera form) match rhymes that would actually sound rhyming when sung in northern speech.
For historical linguists, however, this is the document that nails down the transition from Middle Chinese (隋唐音, c. 600 CE) to early modern Mandarin.
What it proves
- The entering tone (入聲) was gone in northern speech. In the Tang/Song system the fourth tone, 入, was exactly the class of syllables ending in a stop (-p, -t, -k); Zhōngyuán Yīnyùn shows checked syllables already redistributed into the other three tones — exactly the merger that defines modern Mandarin.
- 陰平 and 陽平 had split. The single 平 (level) category of Middle Chinese had divided based on the voicing of the original initial — yielding what we now call tone 1 and tone 2.
- Voiced obstruents had devoiced. The Middle Chinese 全濁 (fully voiced) series — b, d, g, dz, dʒ — was no longer phonemic, the same merger that distinguishes Mandarin from Wu today.
Mandarin from the stage
It is striking that the document fixing Mandarin's birth is a theatre manual, not a court dictionary. Yuan opera was performed for a heterogeneous urban audience — Mongol officials, Han literati, Persian merchants — and demanded a pan-northern register. Zhōngyuán Yīnyùn codifies that register. The 廣韻 (1008) preserved Middle Chinese as a literary ideal; Zhōngyuán Yīnyùn shows what people were actually singing on stage three centuries later.