Silent Periods (SP)
There were two international frequencies designated for emergency use as well as for passing traffic. These were 500 KHz CW (Morse Code) and 2182 KHz Voice (AM and later SSB). For passing traffic a station would use one of these frequencies to make initial contact, then switch to an adjacent working frequency to pass traffic.
However, because high powered signals might mask a weak distress signal being made at the same time, an international system of Silence Periods was enforced.
For 500 KHz the SP was for minute 15, 16 and 17, then again from minute 45, 46 and 47 of every hour.
For 2182 KHz the SP was for minute 0, 1 and2, then again for minute 30, 31 and 32 of every hour.