Vowel harmony
Vowel harmony is pervasive in the languages attributed to Altaic: most Turkic and Mongolic as well as some Tungusic languages have it, Korean is arguably in the process of losing its traces, and it is controversially hypothesized for Old Japanese. (Vowel harmony is also typical of the neighboring Uralic languages and was often counted among the arguments for the Ural–Altaic hypotheses.) Nevertheless, Starostin et al. (2003) reconstruct Proto-Altaic as lacking vowel harmony. Instead, according to them, vowel harmony originated in each daughter branch as assimilation of the vowel in the first syllable to the vowel in the second syllable (which was usually modified or lost later). "The situation therefore is very close, e.g. to Germanic [see Germanic umlaut] or to the Nakh languages in the Eastern Caucasus, where the quality of non-initial vowels can now only be recovered on the basis of umlaut processes in the first syllable." (Starostin et al. 2003:91) The table below is taken from Starostin et al. (2003):