Circle of Fifths
The organizing principle behind keys and scales
Moving by perfect 5ths through all 12 notes reveals deep structure. Five consecutive fifths produce the major pentatonic, seven produce a major mode, and twelve return to the starting note, passing every note in the octave once. Notes adjacent on the circle sound like they are closely related, since the perfect interval between them is highly consonant. If you start a major scale (or any other mode) on adjacent notes on the circle of 5ths, they will share 6 of their 7 notes, making them easy to move between. Harmony often travels along the circle.