Feng Zheng 馮蒸 points out that the characters in the jia group 假攝 generally merge into the guo group 果攝 in nanxi (南戲 southern dramas) and in the rhyme tables of the Song and the Yuan dynasties. The reverse is true in Tang and Song poetry, where characters of the guo group generally merge into the jia group. There is a bipartite development-one promulgated by the central government as standard or literary, and the other used in the common spoken language. The jia group rhymes with the guo group in nanxi as a result of the counterclockwise push-chain shift (逆時針推鏈變化) of the main vowels in the xie 蟹攝, jia and guo groups. The vowel shift began with the second division characters 二等字 of the xie group originally pronounced as diphthong /ai/ which became monophthong /a/. This displaced the characters in the jia group, which were raised to /o/. Due to this new /o/ from the jia group, the characters in the guo group were raised or diphthongized. The jia group temporarily merged with the guo group during the long process during which the vowels in the jia group were raised. The mode of the jia group merging into the guo group in the rhyme tables of the Song and the Yuan dynasties parallels that in the nanxi. These sound changes originated from Southern Chinese. The pronunciations of the guo group and the jia group are different in modern Chinese dialects: they did not merge because the vowel shift above had already occurred. So in most instances, Modern Chinese dialects still have the same number of vowel phonemes among the xie, guo and jia groups after the vowel shift as Middle Chinese dialect did before the vowel shift.