The Cycle of 6ths is a chord progression that I first heard about from the Jazz guitarist Ted Greene, who was a skilled music theorist and had two books published on guitar harmony and chord progressions. Ted stated that he first learned of the cycle of 6ths from studying J.S. Bach. You may be wondering what a chord cycle of ____ is? A chord cycle is when you move chords in a certain intervallic pattern, eventually the cycle comes back around to the starting chord. For instance, if you move chords in 4ths starting on C you would have the following; C, F, Bb, Eb, Ab, Db eventually you move through all 12 keys and come back to C. The cycle of 6ths is a similar idea but you move to relative major or minor of each key. For instance: C major to A minor to F Major to D minor to Bb major and so on, like the other cycle you would eventually come back to C major. I have heard of progressions involving 4ths and 5ths, but never 6ths. John Coltrane does use a cycle of 3rds which would be the inverse pattern of 6ths, but the descending pattern is much harder to find. I hope that some of the information provided here can help shed some light on what they cycle of 6ths is, and why it should be a valuable part of music scholarship
