How did the eastern and western Mongolian language groups develop?

The division between Eastern and Western Mongolian language groups is a subject of ongoing scholarly debate, with many linguists no longer viewing this east-west split as the primary genealogical division. This separation occurred after the divergence of the peripheral, archaizing languages from the central group.1

The Eastern group includes Khalkha, Buryat, and various dialects of Inner Mongolia, while the Western group consists of Oirat and Kalmyk. These groups share numerous linguistic features, such as the loss of initial /h/, the reduction of vowel sequences into long vowels, and the development of rounded vowels in noninitial syllables.12

The primary differences between the two groups are relatively recent innovations. For instance, the Eastern group developed labial harmony, whereas the Western group often retains older forms. While historical and political factors—such as the distinct colonial experiences of the Buryat and Mongol peoples—have influenced their respective vocabularies and scripts, the spoken dialects themselves remain closely related.1