the Turkic speakers' geographical distribution. It mainly pertains to the Southwestern branch of the family. The Codex Cumanicus (12th–13th centuries AD) concerning the Northwestern branch is another early linguistic manual, between the Kipchak language and Latin, used by the Catholic missionaries sent to the Western Cumans inhabiting a region corresponding to present-day Hungary and Romania. The earliest records of the language spoken by Volga Bulgars, the parent to today's Chuvash language, are dated to the 13th–14th centuries AD. Geographical expansion and development With the Turkic expansion during the Early Middle Ages (c. 6th–11th centuries AD), Turkic languages, in the course of just a few centuries, spread across Central Asia, from Siberia to the Mediterranean. Various terminologies from the Turkic languages have passed into Persian, Urdu, Ukrainian, Russian, Chinese, Mongolian, Hungarian and to a lesser extent, Arabic. The geographical distribution of Turkic-speaking peoples across Eurasia since the Ottoman era ranges from the North-East of Siberia to Turkey in the West. (See picture in the box on the right above.) For centuries, the Turkic-speaking peoples have migrated extensively and intermingled continuously, and their languages have been influenced mutually and through contact with the surrounding languages, especially the Iranian, Slavic, and Mongolic languages. This has obscured the historical developments within each language and/or language group, and as a result, there exist several systems to classify the Turkic languages. The modern genetic classification schemes for Turkic are still largely indebted to Samoilovich (1922). The Turkic languages may be divided into six branches: Common Turkic Southwestern (Oghuz Turkic) Southeastern (Karluk Turkic) Northwestern (Kipchak Turkic) Northeastern (Siberian Turkic) Arghu Turkic Oghur Turkic In this classification, Oghur Turkic is also referred to as Lir-Turkic, and the other branches are subsumed under the title of Shaz-Turkic or Common Turkic. It is not clear when these two major types of Turkic can be assumed to have diverged. With less certainty, the Southwestern, Northwestern, Southeastern and Oghur groups may further be summarized as West Turkic, the Northeastern, Kyrgyz-Kipchak, and Arghu (Khalaj) groups as East Turkic. Geographically and linguistically, the languages of the Northwestern and Southeastern subgroups belong to the central Turkic languages, while the Northeastern and Khalaj languages are the so-called peripheral languages. Hruschka, et al. (2014) use computational phylogenetic methods to calculate a tree of Turkic based on phonological sound changes. Schema The following isoglosses are traditionally used in the classification of the Turkic languages: Rhotacism (or in some