this archaic convention was revised, and the boundary between Europe and Asia was now considered to be the Tanais (the modern Don River).
This is the convention used by Roman era authors such as Posidonius, Throughout the Middle Ages and into the 18th century, the traditional division of the landmass of Eurasia into two continents, Europe and Asia, followed Ptolemy, with the boundary following the Turkish Straits, the Black Sea, the Kerch Strait, the Sea of Azov and the Don (ancient Tanais).
To the south, the Kuma–Manych Depression was identified circa 1773 by a German naturalist, Peter Simon Pallas, as a valley that, once upon a time, connected the Black Sea and the Caspian Sea, and subsequently was proposed as a natural boundary between continents.
By the mid-19th century, there were three main conventions, one following the Don, the Volga–Don Canal and the Volga, the other following the Kuma–Manych Depression to the Caspian and then the Ural River, and the third abandoning the Don altogether, following the Greater Caucasus watershed to the Caspian.
The modern border between Asia and Europe remains a historical and cultural construct, defined only by convention.