When the CIS finished its work on 15 June 1920, the actual physical line of the border had not been marked out. This task was not completed until 1921, and was carried out by the Border Marking Commission following instructions from the CIS.

The 68 km border from Flensburg Fjord to the Waddensea was carefully traced. Border markers were placed out in the water and border stones set up on land. As far as possible, the border was allowed to follow the watercourses Kruså, Skelbækken, Gammelå and Sønderå. The border ran along the middle of these watercourses.

This meant that the border followed the slow, natural meanderings of the watercourses. If they changed their beds, a special Border Watercourse Commission stepped in to establish the new boundary. When all the 279 border stones had been laid, a 58-page ‘Border Atlas’ was printed, showing both the land and sea borders between Denmark and Germany on a large scale. The description of the border with the accompanying atlas were approved in Paris on 3 September 1921.

The whole task of marking out the borders between Denmark and Germany was concluded by a treaty on 10 April 1922. This treaty has formed the basis of the Danish-German border inspection ever since.

Every ten years, Danish and German politicians and officials are required together to check whether the border markers in water courses and the border stones on land are in good condition. The last such control took place in 2004.

All the border stones along the old 1864 border were dug up and handed over to Denmark. They were distributed as memorials to places with national monuments, Danish Folk High Schools and museums in Southern Jutland. Two border stones from 1864 stand at the entrance to Sønderborg Castle.