Five by five is the best of 25 possible subjective responses used to describe the quality of communications. As receiving stations move away from an analog radio transmitting site, the signal strength decreases gradually while noise levels increase. The signal becomes increasingly more difficult to understand until it can no longer be heard as anything other than static.
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This is an example of how the system might be used, using British military voice procedure. Station call sign Alpha One wishes to know how well another station, Alpha Two is receiving him.
Alpha One: Alpha Two, Alpha One. How do you read me? Over.
Alpha Two: Alpha One, Alpha Two. I read you five by five. Over