How does Khalkha pronunciation differ from Kalmyk in the word for ‘by horse’?

In the Mongolian languages, the difference in pronunciation for the word meaning "by horse" stems from the development of labial harmony in the Eastern group, which includes Khalkha.

In Khalkha, the vowel a is rounded to o when it follows an o, resulting in the form mor’or. In contrast, Kalmyk—which belongs to the Western group—retains the original a in the suffix, using the form mor’ar.1