Five by Five

From wordorigins.org:

Five-By-Five:

Q: Often in old war movies you'll hear a radio operator say, "I read you five-by-five." What does this mean?

A:The operator in question is ranking the voice transmission on a scale of one to five in two categories, strength and clarity. So, five-by-five is loud and clear and one-by-one would be weak and unintelligible.