From the Wikipedia page 
The Rhine-Neckar Metropolitan Region (German: Metropolregion Rhein-Neckar, pronounced [metʁoˈpoːlʁeˌɡi̯oːn ˌʁaɪnˈnɛkaɐ̯]), often referred to as Rhein-Neckar-Triangle is a polycentric metropolitan region located in south western Germany, between the Frankfurt/Rhine-Main region to the North and the Stuttgart Region to the South-East.
Rhine-Neckar has a population of some 2.4 million with major cities being Mannheim, Ludwigshafen and Heidelberg. Other cities include the former Free imperial cities of Speyer and Worms. The metro area also encompasses parts of the Baden and Palatinate wine regions, the second largest vine region of the country called Deutsche Weinstraße and territory from the three federal states of Baden-Württemberg, Rhineland-Palatinate and Hesse. It has a strong local identity as a successor of the historical Electorate of the Palatinate state.
The region is named after the rivers Rhine and Neckar, which join at Mannheim. Since 2005, the region is officially recognized as a European Metropolitan Area.