It was formed from the northern part of the French Occupation Zone , ( the Rhenish Palatinate or which had been a part of Bavaria ) as well as the southern parts of the Prussian Rhine Province , Birkenfeld which formerly belonged to Oldenburg ) , parts of the Prussian Province of Nassau ( see Hesse-Nassau ) , and parts of Hesse-Darmstadt as well as Rheinhessen on the western banks of the Rhine ) . It was formed from the northern part of the French Occupation Zone , which included parts of Bavaria ( the Rhenish Palatinate ) , the southern parts of the Prussian Rhine Province ( including the District of Birkenfeld which formerly belonged to Oldenburg ) , parts of the Prussian Province of Nassau ( see Hesse-Nassau ) , and parts of Hesse-Darmstadt ( Rhinehessen on the western banks of the Rhine ) ; the constitution was legally confirmed by referendum on 18 May 1947 .