Cycle of fifths
From DNBWiki
The Cycle of fifths (or circle of fifths) is a concept in western music that is based on the fact that by repeatedly ascending a perfect fifth and sometimes dropping down an octave it is possible to reach every note in the twelve semi-tone chromatic scale and return to the first.
[edit] Pythagorus
Pythagorus is said to have discovered that notes make a good sound together when they're frequencies are in an integer ratio. By increasing the frequency by a ratio of 3:2, a natural perfect fifth is heard. By increasing a frequency by a fifth, twelve times, then transposing down seven octaves, you end up back at the same note except for a small discrepancy known as the pythagorean comma.
