18th-century definitions drew the boundary north of the Caucasus, across the Kuma–Manych Depression. This definition remained in use in the Soviet Union during the 20th century.
In western literature, the continental boundary has been drawn along the Caucasus watershed since at least the mid-19th century.
See e.g. Baron von Haxthausen, "Transcaucasia" (1854); review Dublin university magazine
Douglas W. Freshfield, "Journey in the Caucasus", Proceedings of the Royal Geographical Society, Volumes 13–14, 1869.
