Before scientific exchanges could get off the ground, the U.S. and Soviet governments had to set some ground rules about who could go where. Oddly enough, the Soviets had the upper hand in this: in March 1953 (shortly after the death of Stalin), the Communist nation decided to allow Americans to enter certain parts of the country. The U.S.’s willingness to admit visitors, meanwhile, was limited by a 1952 law that barred the admission of all Communists. (Of course not all Soviet citizens were Communists, but American foreign policy officials assumed that any Soviet citizen allowed to come to the United States was a member of the party.)
A National Security Directive issued in January 1955 outlined a solution: the U.S. would open its borders to Soviet visitors, but only on principles of strict reciprocity. If Americans could visit 70 percent of Soviet cities with populations over 100,000, then Soviets could visit 70 percent of American cities with populations over 100,000.
You can see the map, and a longer explanation, in Slate.
P.S. It turns out that almost all online comments on this map focused on “ICBM missile bases in South Dakota.” The problem? There were no ICBM missile bases in South Dakota in 1955, because ICBMs did not yet exist. More in BoingBoing!