Proto-Turkic phonology

Proto-Turkic exhibited vowel harmony, a feature sometimes also ascribed to Proto-Altaic, distinguishing vowel qualities e, i, o, u vs. ë, ï, ö, ü besides a, as well as two vowel quantities.

The consonant system had a two-way contrast of stop consonants (fortis vs. lenis), k, p, t vs. g, b, d, with verb-initial b- becoming h- still in Proto-Turkic. There was also an affricate consonant, č; at least one sibilant s; and sonorants m, n, ń, ŋ, r, ŕ, l, ĺ with a full series of nasal consonants.

The sounds denoted by ń, ĺ, ŕ refer to palatalized sounds and have been claimed by Altaicists to be direct inheritances from Proto-Altaic. The last two can be reconstructed with the aid of the Oghur languages, which show for *ŕ, *ĺ, while Common Turkic has *z, *š. Oghuric is thus sometimes referred to as Lir-Turkic and Common Turkic as Shaz-Turkic.

However, an alternate theory holds that Common Turkic is closer to the original state of affairs, and reconstructs Proto-Turkic *z, *š. The glottochronological reconstruction based on analysis of isoglosses and Sinicisms points to the timing of the r/z split at around 56 BCE–48 CE. This, as A. V. Dybo puts it may be associated with:

"the historical situation that can be seen in the history of the Huns' division onto the Northern and Southern [groups]: the first separation and withdrawal of the Northern Huns to the west has occurred, as was stated above, in 56 BC,...the second split of the (Eastern) Huns into the northern and southern groups happened in 48 AD."

Dybo suggests that during that period, the Northern branch steadily migrated from western Mongolia through southern Xinjiang into the north's Dzungaria and then finally into Kazakhstan's Zhetysu until the 5th century.