Dissolution of Czechoslovakia
Since the federalization in 1968, Czechoslovakia had divided citizenship, either of the Czech Socialist Republic or of the Slovak Socialist Republic (the word 'Socialist' was dropped from both names shortly after the Velvet Revolution). This distinction, however, had little effect on the citizen's life. On January 1, 1993, all Czechoslovak citizens automatically became citizens either of the Czech Republic or the Slovak Republic, based on their previous citizenship, permanent residence address, birthplace, family ties, job, and other criteria. Additionally, people had one year's time to claim the other citizenship under certain conditions.
Postage stamps and postal history of Slovakia
After World War I and the dissolution of the Austro-Hungarian Empire, Slovakia and the regions of Bohemia, Moravia, and Carpathian Ruthenia formed the Czechoslovak Republic. From 1918 to 1939, stamps of the Czechoslovak Republic inscribed either "Česko-Slovensko" or "Československo" were used in Slovakia. 
Between 1939 and 1945, the Slovak Republic, a client state of Nazi Germany, issued stamps inscribed "Slovensko".