The Circle of 5ths (or Cycle of 5ths) represents how the relationships between the 12 possible major keys (and their enharmonic spellings) make up the chromatic scale when moving down one perfect fifth from each key. It is also possible to go the other direction which moves in Perfect 4ths. Moving in Perfect 4ths creates very strong bass movement.
So what does this do for us? The circle of fifths provides us a quick visual reference to a lot of information about all 12 major keys in western music, and helps us see tonal relationships between the keys.
If you were to draw a straight line through the circle (such as Eb to A, or G to Db) you have the relationship of a "tri-tone" (the building block of a dominant 7th chord). By going counter clockwise (circle of 4ths), it shows you which dominant 7th chords resolve to what tonic (example, G7 to C, or F to Bb). Any right angle (90 degrees) drawn on the circle will show the relative minor (such as C to Amin, or F to Dmin). Chord tones and chord progressions can be easily transposed by rotating your imaginary lines or triangle drawn on the circle (example: draw a triangle to show a C Major triad - C, E, G. Then rotate that triangle to the right one degree and it will display a G Major triad - G, B, D).
Being a "circle" shows by starting at any key and cycling all the way through the circle of 5ths or circle of 4ths you will arrive back at the original key. It also serves as a clean way to see the order of sharps and flats in each key.
Also known as the Circle of 5ths, Circle of Fifths, Cycle of 5ths, Cycle of Fifths, Circle of Fourths, Circle of 4ths, Cycle of Fourths, and Cycle of 4ths.