CONELRAD stands for CONtrol of ELectronic RADiation. Introduced by President Truman
in 1951, this was America's first national alerting system.
In the event of a Soviet attack on the United States, all commercial radio stations, and ham radio operators were required to cease transmission. This was to prevent Soviet bombers from homing in on their targets by using specific broadcast and amateur transmitters as navigation beacons. Instead, selected CONELRAD stations would broadcast on either 640kHz or 1240kHz to inform the public about emergency measures.
Heathkit sold a monitor for ham operators that would automatically disconnect power from their transmitters in the event of a CONELRAD alert (ie when the carrier signal from a local non-CONELRAD station was dropped).
As part of the system it was obligatory for all radios sold after 1953 to have the CONELRAD frequencies 640/1240 kHz marked with small triangles on the dial. The triangles were known as CD marks, for Civil Defense. This requirement was dropped when the CONELRAD system was replaced by the Emergency Broadcast System in 1963. With the USSR's introduction of Intercontinental Ballistic Missiles, however, CONELRAD became impractical due to the decrease in response time from heavy bombers to the ICBMs.