Which features are shared by Eastern and Western Mongolian languages?

The Eastern and Western Mongolian language groups share numerous linguistic features, which has led many contemporary linguists to move away from viewing the east-west split as the primary genealogical division of the Mongolian languages.

Shared features between these groups include:

• Loss of initial /h/: The original initial /h/ sound has been lost in both groups.1
• Vowel reduction: Sequences of vowels have been reduced to single, long vowels.1
• Rounded vowels: Both groups exhibit the development of rounded vowels in noninitial syllables.1
• Vowel assimilation: There is a shared tendency for the assimilation of /i/ to the vowel of the following syllable.1

While these groups share these fundamental characteristics, they do exhibit some differences, such as the Eastern group's development of labial harmony—a relatively recent innovation where, for example, the vowel a is rounded to o following an o in the preceding syllable.1