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In page Duchy of Schleswig:

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The Treaty of Versailles provided for plebiscites to determine the ownership of the region.[1] Thus, two referenda were held in 1920, resulting in the partition of the region. Northern Schleswig voted by a majority of 75% to join Denmark, whereas Central Schleswig voted by a majority of 80% to remain part of Germany. In Southern Schleswig, no referendum was held, as the likely outcome was apparent. The name Southern Schleswig is now used for all of German Schleswig. This decision left substantial minorities on both sides of the new border.