The Ruhr is a tributary of the Rhine, which has given its name to the surrounding conurbation in the western German state of Nord Rhein-Westfalen. In the late 19th and early 20th Century it developed into one of Germany's primary zones for heavy industrial production. Its industries include coal mining, iron and steel manufacture, and heavy engineering production. Major centres of the Ruhr include Bochum, Dortmund, Duisburg, Essen, and Gelsenkirchen. The conurbation has a population of 5.3 million.
The conurbation takes its name from the River Ruhr, which roughly marks the southern boundary of the district (which extends westwards to the Rhine and northwards to the River Lippe). The name Ruhr is believed to derive from an Indo-European root word reu, meaning 'to hollow out'.
The Ruhr was very heavily bombed during the Second World War.