Old Turkic (also East Old Turkic, Orkhon Turkic language, Old Uyghur) is the earliest attested form of the Turkic languages, found in Göktürk and Uyghur Khaganate inscriptions dating from about the eighth to the 13th century. It is the oldest attested member of the Siberian Turkic branch of Turkic, which is extant in the modern Western Yugur language. It is not the ancestor of the Uyghur language; the contemporaneous ancestor of Uyghur is called Middle Turkic, later Chagatai or Turki. Old Turkic is attested in a number of scripts, including the Old Turkic script, the Old Uyghur alphabet (a form of the Sogdian alphabet), the Brahmi script, and the Manichaean script. Old Turkic often refers not to a single language, but collectively to the closely related and mutually intelligible stages of various Common Turkic languages spoken during the late first millennium. Sources The sources of Old Turkic are divided into two corpora: the 8th to 10th century Orkhon inscriptions in Mongolia and the Yenisey basin (Orkhon Turkic, or Old Turkic proper). 9th to 13th century Uyghur manuscripts from Gansu and Xinjiang (Old Uyghur), in various scripts including Brahmi, the Manichaean, Syriac and Sogdian alphabets, treating religious (Buddhist, Manichaean and Church of the East), legal, literary, folkloric and astrological material, as well as personal correspondence. Writing systems The Old Turkic script (also known variously as Göktürk script, Orkhon script, Orkhon-Yenisey script) is the alphabet used by the Göktürks and other early Turkic khanates during the 8th to 10th centuries to record the Old Turkic language. The script is named after the Orkhon Valley in Mongolia where early 8th-century inscriptions were discovered in an 1889 expedition by Nikolai Yadrintsev. This writing system was later used within the Uyghur Khaganate. Additionally, a Yenisei variant is known from 9th-century Yenisei Kirghiz inscriptions, and it has likely cousins in the Talas Valley of Turkestan and the Old Hungarian alphabet of the 10th century. Words were usually written from right to left. Variants of the script were found from Mongolia and Xinjiang in the east to the Balkans in the west. The preserved inscriptions were dated to between the 8th and 10th centuries. Phonology Vowel roundness are assimilated thorough the word through vowel harmony. Some vowels were considered to occur only in the initial syllable, but they were later found to be in suffixes. Length is distinctive for all vowels; while most of its daughter languages