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To increase safety at sea and in the air, communications procedures were developed for use in times of distress. The international distress frequency is 500 KHz. In routine communication, merchantmen contacted each other (or shore stations) on 500 KHz and then shifted to a working frequency. To make sure that routine traffic does not interfere with distress traffic, two SILENT PERIODS were designated. These periods of 3 minutes each were from 15 to 18 minutes and 45 to 48 minutes past each …

…CW frequency of 500 kHz [using RT, of course]. In 1932, the International Telecommunications Convention of Madrid established 1650 kHz as the international RT distress frequency. Individual countries could decide when to adopt its use [i.e. instead of 500 kHz RT]. Many countries were still using 500 kHz distress RT up to the 1940s. 1650 kHz remained the RT distress freq. right up until 1950 when it was replaced by 2182 kHz as decreed by the International Telecoms Convention of Atlantic City 1947.