standards. In all of the participating countries Class I and II stations were exclusively assigned to Clear Channel frequencies, while Class III was synonymous with a Regional frequency assignment. In the United States, Class IV stations were only assigned to Local frequencies, although in other countries they were assigned to both Local and Regional ones. A major change was the provision that some clear channels were allocated to be used simultaneously by two stations — those maintaining sole use of a frequency were classified as Class I-A, while stations sharing a clear channel were known as Class I-B. The Agreement assigned six Class I-A frequencies each to Mexico and Canada, and one to Cuba. Reflecting the existence of improved radio design, the Agreement also reduced the "same market" minimum frequency separation from 50 to 40 kHz. (Mexico elected to further adopt a 30 kHz "same market" spacing, unless this was in conflict with an adjoining nation's "border zone" allocations.) This closer spacing was particularly important in the case of the two highest Local frequencies, 1420 and 1500 kHz, as stations on these frequencies were being moved to 1450 and 1490 kHz, a 40 kHz separation. According to the Agreement's provisions, its implementation was to take place within one year after its adoption by the pact's four main signatories — the United States, Canada, Cuba and Mexico. Cuba was the first to ratify, on December 22, 1937, and was followed by the U.S. on June 15, 1938 and Canada on November 29, 1938. While waiting on Mexico, in 1939 the U.S. and Canada completed a frequency agreement based on the treaty standards. Mexico finally approved the treaty on December 29, 1939, and work commenced on adopting its wide-ranging provisions. March 29, 1941 implementation An engineering conference, with representatives from the U.S., Canada, Cuba, the Dominican Republic and Mexico, was held from January 14–30, 1941 in Washington, D.C., in order to coordinate the upcoming changes. With a few exceptions the frequency shifts were scheduled to be implemented at 0800 Greenwich Mean Time (3 a.m. E.S.T.) on March 29, 1941, which was informally known as "moving day". (Philadelphia stations petitioned mayor Robert Lamberton to declare a "Radio Moving Day", but he refused on the grounds that "My experience has been that proclamations by the mayor mean just exactly nothing and I issue as few as I can.") The frequency changes affected "about a