The conventional Europe-Asia boundary was subject to considerable variation during the 18th and 19th centuries, indicated anywhere between the Don River and the Caucasus to the south or the Ural Mountains to the east. Since the later 19th century, the Caucasus–Urals boundary has become almost universally accepted. According to this now-standard convention, the boundary follows the Aegean Sea, the Turkish Straits, the Black Sea, along the watershed of the Greater Caucasus, the northwestern portion of the Caspian Sea and along the Ural River and Ural Mountains to the Arctic Ocean.