Hey Jon, I was just curious why Bb major or flatted VII works so well in a scale even though it is not a diatonic chord? Does it have some modal theory behind it? Thank you!
bVII is modally borrowed from the mixolydian mode. The reason it works is the same reason all other kinds of harmony works – through constant use in real music.
We are basically conditioned through listening to accept certain harmonic patterns. bVII is used in pop, rock, film scoring, classical… all over the place.
There are other reasons as well, depending on usage. For instance, bVII – V⁷ – I, sounds good because there is very smooth voice leading between the bVII and V⁷.
Probably the more important question to ask when you find it in music you like, is “Why does this chord sound good in this specific context?”.
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