Under Articles 109 – 114 of the Treaty of Versailles, an International Commission (the Commission Internationale de Surveillance du Plébiscite Slesvig – CIS) was set up and given responsibility for holding referendums in Schleswig.

The commission comprised four members – one each from Great Britain, France, Sweden and Norway – and took over the administration of the two referendum zones in Schleswig on 26 January 1920.

German troops and public officials were required to leave the two zones and CIS took responsibility for maintaining public order using its own police force and contingents of French and British troops.

Special CIS postage stamps were issued to indicate that the referendum zones were an area under special administration.

CIS kept a close eye on the referendum campaigns and the two votes were held in an orderly fashion on 10 February (in Zone 1) and 14 March (in Zone 2).

The commission then prepared a proposal for the new border on the basis of the referendum results.

Its mandate expired on 15 June 1920, after which Zone 1 became part of Denmark and Zone 2 was returned to Germany.