The Chuvash language is the only descendant of the Ogur Turkic language variety, which separated from the Common Turkic language unity ca. 2000 years ago. The speakers of this Turkic language variety appeared in Eastern Europe in the 5th century. Inhabiting the steppe zone they established political, cultural and language contacts with the neighbouring peoples. In the 9th century some of them moved to the Volga-Kama confluence, the territorial varieties of their language known as Volga Bulgarian became dominant between the 9th and 13th centuries. Due to the Mongol invasion after 1236 only one dialect of Volga Bulgarian was preserved, on the basis of which the Chuvash language has emerged.0In the book of Klára Agyagási, the processes of Chuvash historical phonetics are reconstructed relying on data from various language contacts as oral sources: lexical copies from Ogur, Volga Bulgarian into Ancient Hungarian, Proto-Permian, Old Russian, Proto-Mari and Middle Kipchak, as well as copies from Arab, New Persian, Proto-Permian, Old and Middle Russian, Chinese, Middle Mongolian, Proto-Mari, the Low Cheremis substratum and Middle Kipchak into Chuvash. As a result, the author presents the first comprehensive historical phonetics of the Chuvash language arranged in chronological order, applying the code-copying and areal linguistic framework.