The Duchy of Oldenburg was a historical territory located in northwestern Germany. It existed from the early 12th century until 1918, when it became part of the Free State of Oldenburg following the end of monarchies in Germany. The duchy was known for its agricultural economy and was ruled by the House of Oldenburg, which also produced several European monarchs. The capital of the duchy was the city of Oldenburg, which served as a cultural and administrative center. The region played a significant role in the political landscape of northern Germany and was involved in various alliances and conflicts throughout its history.