Bithynia was an ancient region, kingdom and Roman province in the northwest of Asia Minor, adjoining the Sea of Marmara, the Bosporus, and the Black Sea. It bordered Mysia to the southwest, Paphlagonia to the northeast along the Pontic coast, and Phrygia to the southeast towards the interior of Asia Minor.

Nicomedia was an ancient Greek city located in what is now Turkey. In 286, Nicomedia became the eastern and most senior capital city of the Roman Empire, a status which the city maintained during the Tetrarchy system (293–324).