One of the most emphasized issues of the Turkish language, which also causes differences of opinion among Turcologists,is the state of the sound / y- / at the beginning of the words.This sound varies in terms of both historical Turkish dialects and contemporary Turkish dialects and is met with a different sound. For those who accept the Altaic hypothesis, it is known that the Proto-Turkic language, which developed as an independent language after the era of the Proto- Altaic language, was divided into two branches as Oghur languages (West Old Turkish) and Common Turkic (East Old Turkish). In Turcology, the terms Lir Turkic for Oghur languages and Shaz Turkic for Common Turkic are also used. In order to classify the Turkic languages, the sounds /l, /ş/ and /r/, /z/ sounds, and also the / y- / sound at the beginning ofthe words were used as criteria. This article will focus on thedevelopment of the / y- / sound at the beginning of the words in Turkic languages, which corresponds to the / j- / sound at the beginning of the words in the Modern Kazakh language through diachronic and synchronic comparisons. It will be mentioned that whether the /y-/ sound can be seen since the Orkhon Turkic, which is one of the historic periods of the East Old Turkic, or the /j-/ sound in the Kazakh language is more archaic.