The Rhine (German: Rhein; Dutch: Rijn; French: Rhin; Italian: Reno; Romansh: Rain; Latin: Rhenus) is one of the longest and most important rivers in Europe at 1,320 kilometres (820 mi), with an average discharge of more than 2,000 cubic meters per second. The name of the Rhine comes from the archaic German Rhine, which in turn comes from Middle High German: Rin, from the Proto-Indo-European root *reie- ("to flow"). The Reno River in Italy shares the same etymology.
The Rhine and the Danube formed most of the northern inland frontier of the Roman Empire, and since those days the Rhine has been a vital navigable waterway, carrying trade and goods deep inland. It has also served as a defensive feature, and has been the basis for regional and international borders. The many castles and prehistoric fortifications along the Rhine testify to its importance as a waterway. River traffic could be stopped at these locations, usually for the purpose of collecting tolls, by the state controlling that portion of the river.