A dual monarchy is when one monarch rules over two self-governing kingdoms, sharing government structures like the military and foreign policy. The term usually refers to Austria-Hungary, which existed from 1867 to 1918.
The former realm of Austria-Hungary, of 1867-1918, consisted of land from modern-day Austria, Hungary, the Czech Republic, Slovakia, Slovenia, Croatia, Bosnia-Herzegovina, Poland, Romania, Ukraine and Italy. The dual monarchy was split up after World War I, and the land was divided. A new country, Czechoslovakia, was created from some of this land after the war, which was again divided in 1993 into present-day Slovakia and the Czech Republic.