The most powerful and most used progression in music is dropping a 5th in the root. Anyone has merely to play a Dominant chord of a key followed by the 1st chord or Tonic of that key to understand why the monks of Europe in the Dark Ages named the 5th chord of a key Dominant. It is an inherent rule of music that the Dominant chord leads to the Tonic. Anyone can hear the settling of the tone center when this particular chord change is played.
For this particular example of chord substitution I’m going to use an E7 going to an A7 resolving to a Dmaj7. The A7 chord in this progression is going to be substituted by an Eb7 chord which is the same chord type an augmented 4th away in the root. You can hear in the example that this is a pleasing similar sound. I don’t know why this works but it does. It also happens that the 3rd’s and 7th’s of the A7 and the Eb7 are the same notes reversed in position, making the different chords quite alike. What you can realized out of this harmonic device is a chromatically descending chord progression that is different from the cycle of 5th’s with the same amount of solidity and power. When using this device you must adjust the chords to reflect the notes of the melody that are happening when the chord substitution is being played.
