Five by five
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Five by five is the best of 25 possible subjective responses used to describe the quality of communications, specifically the signal-to-noise ratio. As receiving stations move away from an analog radio transmitting site, the signal strength decreases gradually, causing the relative noise level to increase. The signal becomes increasingly difficult to understand until it can no longer be heard as anything other than static.
In voice procedure (the techniques used to facilitate spoken communication over two-way radios) a transmitting station may request a report on the subjective quality of signal they are broadcasting. In the military of the North Atlantic Treaty Organization (NATO) countries, and other organizations, the signal quality is reported on two scales; the first is for signal strength, and the second for signal clarity or 'readability.' Both these scales range from one to five, where one is the worst and five is the best. The listening station reports these numbers separated with the word "by". "Five by five" therefore means a signal that has excellent strength and perfect clarity — the most understandable signal possible.
"Five by five" (occasionally written "'5 by 5", "five-by-five", "5 × 5", or "5-by-5"), by extension, has come to mean "I understand you perfectly" in situations other than radio communication. Further shortened forms are "five by", "Fivers" and "Fifers". Post-World War II, the phrase "Loud and Clear" entered common usage with a similar meaning.
The term is arguably derived from the signal quality rating systems such as shortwave's SINPO code or amateur radio's RST code. Given that this slang spans not only generations but also a spectrum of communications technologies (spark-gap transmitters, shortwave, radio telephone, Citizen's Band (CB) radio, cellular among others) and organizations (hobbyist, commercial, many military branches in different countries), there are many interpretations in popular misuse.
This reporting system is not appropriate for rating digital signal quality. This is because digital signals have fairly consistent quality as the receiver moves away from the transmitter until reaching a threshold distance. At this threshold point, sometimes called the "digital cliff," the signal quality takes a severe drop and is lost. This difference in reception reduces attempts to ascertain subjective signal quality to simply asking "Can you hear me now?" or similar. (The only possible response is "Yes;" otherwise, there is just dead air.) This sudden signal drop was also one of the primary arguments of analog proponents against moving to digital systems. However, the "five bars" displayed on many cell phones does directly correlate to the signal strength rating.
Cultural references 
In the video game Deus Ex, a password for a computer is 5X5.
In the Blizzard computer game StarCraft, the Terran dropship unit quotes "In the pipe, five by five" when confirming a move order. The unit portrait of the Terran dropship also heavily resembles the Aliens character Corporal Collette Ferro.
In the video game Call of Duty: Modern Warfare 3, SEAL Delivery Vehicle Team 4 (SDV Team 4) uses it as a radio check respond to Sandman (Metal 0-1), "Roger, 0-1. We have you five-by-five. Phase line Echo secured. We have execute authority."
See also 
- Imel & Hart 2003, p.38.
- Imel. Kathy J. & Hart, James W., P.E. Understanding Wireless Communications in Public Safety: A Guidebook to Technology, Issues, Planning, and Management for the National Law Enforcement and Corrections Technology Center (Rocky Mountain Region) Plain text
- Ham Radio RST Signal Reporting System for CW Operation, by Charlie Bautsch, W5AM