How did Common Turkic differ from Bolgar Turkic?

Common Turkic and Bolgar Turkic represent a fundamental early split within the Turkic language family. The primary distinction between these two groups lies in regular phonetic representations, specifically the correspondence of r versus z and l versus š.1

Examples of these phonetic differences include:
* Hundred: Chuvash śer (Bolgar) versus Turkish yüz (Common).1
* Age: Chuvash śul (Bolgar) versus Turkish yaš (Common).1

While Common Turkic encompasses the majority of modern Turkic languages, Bolgar Turkic is represented today solely by Chuvash, which originated in Volga Bolgarian and retains archaic features. Due to these historical divergences, Chuvash and Common Turkic languages are not mutually intelligible.12