I’d heard the term “the circle of fifths” for years without ever really seeing the principle in action.   It is a very cool principle.   Hearing the fifth note of a scale makes the ear want to go back to the first note, the fifth note demands to be resolved to the one in Western Music.  Instead of the one,  play the five of that one, then the five of that one, and repeat, and repeat and repeat, in a perfect circle.  

It’s kind of cool to watch it in action, to play it through yourself a few times.  I saw a chart of the circle of fifths recently, and a guitarist gave a good lesson about the uses of this circle of fifths we have heard so much about over the years without learning what it actually was until the other day.  The chord structures of every melody we know are based on this circle of fifths, which is also a circle of fourths, it turns out.

Clockwise this circle keeps going up a fifth, a perfect circuit of fifths.  Go the other way and the chords move by fourths.   The relative minor variations of these are also fun to mess around with, trying to play the cycle in a nice smooth circle.