Treaty on the Creation of the USSR
In 1917, Russia restricted emigration by instituting passport controls and forbidding the exit of belligerent nationals. In 1922, after the Treaty on the Creation of the USSR, both the Ukrainian SSR and the Russian SFSR issued general rules for travel that foreclosed virtually all departures, making legal emigration impossible. Border controls thereafter strengthened such that, by 1928, even illegal departure was effectively impossible. This later included internal passport controls, which when combined with individual city Propiska ("place of residence") permits, and internal freedom of movement restrictions often called the 101st kilometre, greatly restricted mobility within even small areas of the Soviet Union.