(Dutch: Rijn; French: Rhin; German: Rhein; Italian: Reno; Latin: Rhenus; Romansh: Rain) is one of the longest and most important rivers in Europe, at about 1,232 km (766 mi), with an average discharge of more than 2,000 m3/s (71,000 cu ft/s).
The name of the Rhine derives from Gaulish Renos, and ultimately from the Proto-Indo-European root *reie- ("to move, flow, run"), which is also the root of words like river and run. The Reno River in Italy shares the same etymology. The spelling with -h- seems to be borrowed from the Greek form of the name, Rhenos, seen also in rheos, stream, and rhein, to flow.
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 and 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 that controlled that portion of the river.
Until 1932 the generally accepted length of the Rhine was 1,230 kilometres (764 miles), but in 1932 the German encyclopedia Knaurs Lexikon stated its length as 1,320 kilometres (820 miles), presumably through a typographical transposition error. This number was then copied the next year in the authoritative Brockhaus Enzyklopädie; apparently no one spotted the mistake, and the new number became generally accepted, finding its way into text books and official publications. Only in 2010 did Bruno Kremer of the University of Cologne notice the discrepancy between the old and then current values; on further investigation he realized that the accepted value of 1320 km was in error. His findings have been checked and confirmed by the Dutch Rijkswaterstaat, who determined the length to be 1,232 kilometres (766 miles).