Fromthe Wikipedia page 
Rhein-Neckar-Kreis is a Landkreis (district) in the northwest of Baden-Württemberg, Germany. Neighboring districts are Bergstraße, Odenwaldkreis, Neckar-Odenwald, Heilbronn, Karlsruhe, the independent city of Speyer, the Rhein-Pfalz-Kreis, and the independent cities of Mannheim and Heidelberg. The administrative headquarters are based in the city of Heidelberg, which does not form part of the district itself.
The district was created in 1973 by merging the previous districts of Heidelberg, Mannheim and a big part of Sinsheim.