Previous chapters dealt with the Proto-Turkic sound structure and its subsequent development. Numerous phenomena mentioned above will be reconsidered in a diachronic perspective, with renewed reference to some data that are worthy of repetition. As noted in § 3.2, Proto-Turkic stands for the earliest reconstructible hypothetical stage, a common ancestor language, existing prior to the separation of Common Turkic and Oghur Turkic, long before the stage mirrored in the oldest available sources (Róna-Tas 1998b: 69–72; Johanson 1998a: 89–106). Old Turkic is certainly not identical to Proto-Turkic, not even chronologically close to it, but it nonetheless provides the most valuable material for reconstructions.