In 1891 Danish historian Hans Victor Clausen (1861-1938) published a map in his book ”Landmandsbogen” as an attempt to establish a cultural dividing line in Sønderjylland (South Jutland), which was a disputed region between Germany and Denmark, called ”Sprog- og Sindelagsgrænse” ("language and attitude border"). The so called "Clausen Line" proved to be almost identical to the distribution of the German and Danish languages, that he also documented. The language-barrier ran in a west-east direction south of Tønder and ended north of Flensburg in the Flensburg Fjord.
He also noted that German was spoken predominantly in the towns of Sønderjylland and in Flensburg. However, the colloquial language to the north and the south of the Clausen Line was in fact either mostly sønderjysk (Low Danish) or Low German.
In connection with the 1920 plebiscite, the voting zones were determined according to his proposals. In the northern zone, 75 % voted for Denmark and 25 % for Germany. In the southern zone together with Flensburg, 80 % voted for Germany and 20 % for Denmark. Apart from minor changes, the German-Danish state border has run along the "Clausen Line" ever since.
