The eastern border of Anatolia has been held to be a line between the Gulf of Alexandretta and the Black Sea, bounded by the Armenian Highlands to the east and Mesopotamia to the southeast. By this definition Anatolia comprises approximately the western two-thirds of the Asian part of Turkey. Today, Anatolia is sometimes considered to be synonymous with Asian Turkey, thereby including the western part of Armenian Highland and northern Mesopotamia; its eastern and southern borders are coterminous with Turkey's borders.