There are hundreds of local Chinese language varieties forming a branch of the Sino-Tibetan language family, many of which are not mutually intelligible. Variation is particularly strong in the more mountainous southeast part of mainland China. The varieties are typically classified into several groups: Mandarin, Wu, Min, Xiang, Gan, Jin, Hakka and Yue, though some varieties remain unclassified. These groups are neither clades nor individual languages defined by mutual intelligibility, but reflect common phonological developments from Middle Chinese.