A hundred & one thrilling trips around the Cycle of 5ths
The so called "Cycle (or Circle) of Fifths (or Fourths)" is one of the primary building blocks, melodically and harmonically, of what's become the basis of Western Music.
Derived from the natural harmonic series, the interval of a Fifth is the first overtone, or partial, that occurs after the octave, (i.e. C - C - G......). The Fifth has been "instrumental", with the advent of equally tempered tuning, in forming the Western sense of pitch, and ultimately - harmony.