Early classifications of varieties of Chinese, such as those of Li Fang-Kuei in 1937 and Yuan Jiahua in 1960, divided Min into Northern and Southern subgroups.
However, in a 1963 report on a survey of Fujian, Pan Maoding and colleagues argued that the primary split was between inland and coastal groups.
In a reclassification that has been followed by most dialectologists since, they restricted the term Northern Min to inland dialects of Nanping prefecture, and classified the coastal dialects of Fuzhou and Ningde as Eastern Min.
Although coastal Min varieties can be derived from a proto-language with four series of stop or affricate initials at each point of articulation (e.g. /t/, /tʰ/, /d/ and /dʱ/), Northern Min varieties contain traces of two further series, one voiced and the other voiceless.
In Northern Min dialects, these initials have a different tonal development from other stops and affricates, though the details vary between varieties.
Moreover, although in Jian'ou and Zhenghe these initials yield voiceless unaspirated initials (as in coastal varieties), they yield voiced sonorants or the zero initial in Jianyang and Wuyishan.
Because of these reflexes, Jerry Norman called these initials "softened" stops and affricates.
Fujian Normal University Research Institute 福建师范学院. n.d. Mindong, Bei fangyan diaocha ziliao huibian (part 1) 闽东、北方言调查资料汇编: 第1辑. Fujian Normal University Research Institute 福建师范学院, Dialectology group, Chinese language department 中文系语言教研组 方言调查小组.
Huang Chin-wen 黄金文. 2001. 方言接觸與閩北方言演變 / Language contact and the phonological changes in North Min. Taipei: National Taiwan University.
Ma Chongqi 馬重奇. 2014. 明清閩北方言韻書手抄本音系研究 / A phonological study of the manuscripts of Northern Fujian Dialect rhyme books in the Ming and Qing Dynasties. Beijing: Commercial Press. ISBN978-7-100-07351-6