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In page Sauerland:

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The name Sauerland is first mentioned as Suderland in an official document from 1266.[1] After 1400 the letter 'd' started to disappear. Therefore, Sauerland = southern country is the most convincing meaning, opposed to the theory that Sauer is from the German word sauer meaning sour (poor "sour" soil). Linguistically, "suder-“ is similar to the Old Saxon sûðar (southbound).