Mel Bay Circle of Fifths for Guitar Book, Mel Bay Publishing

"The Circle of Fifths is a schematic diagram of the well-tempered tonal system. The 12 major keys and their parallel minors are arranged around the circle like the numbers denoting the 12 hours on a clock. The root notes of neighboring keys are separated by the interval of a fifth, clockwise a fifth up and counterclockwise a fifth down. The AMA Circle of Fifths emphasizes important harmonic interrelationships that can be easily recognized using the wheel."