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Originally posted by Finn mac Cumhal:
This phrase refers to military jargon used in radio communication. At one time (about 50 years ago) the radio receiver had two dials with 5-point graduations that marked the strength of incoming signal and clarity of transmission received. If the operator was receiving the transmission at the highest measured values, the operator would tell the pilot that the signal was “five by five.” Less well-received signals could be indicated by variations, e.g. “three by four.”

The phrase “coming in loud and clear” also has its origin with early military communication.

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