Trier was one of the three Regierungsbezirke of Rhineland-Palatinate, Germany, located in the west of the state. The region was created in 1815 as part of the Prussian Rhineland. Until 1920 the bulk of what then became the Territory of Saar Basin was part of the Trier Region. Only the east of the Saar Territory, today's Saar-Palatinate district, formerly the western fringes of the then Bavarian Rhenish Palatinate, was previously no part of the Trier Region. In April 1937 the St. Wendel-Baumholder district, formed in 1920 from those parts of the St. Wendel district that had not been seceded to the Saar Territory, was dissected from the Trier Region and merged in the new Birkenfeld district within the Koblenz Region.