5 x 5

I learned tonight that the expression "five by five" has it's roots in radio. I have heard people use this expression when they mean "good" (for instance, the Space Marine piloting the landing craft in Aliens announces to the crew of final decent "We are in the pipe five by five").

The short answer is that 5 x 5 is the best possible rating for loudness and clarity. The long story is more involved, but it still amounts to "loud and clear".