Ezra George on Instagram: "The crux of the backdoor progression is the bVII7 chord, which is borrowed from the parallel minor key. Jerry Coker gave popularity to the name in his book “Elements of the Jazz Language for the Developing Improvisor” published in 1997. The bVII7 chord, as well as other unconventional dominant chords, have been incorporated and justified through the Barry Harris method, which relies heavily on the symmetrical structure of fully diminished seventh chords."