I this post, I received some information on the usage of a flattened-seventh chord in progressions, which, along with the flattened-third, seem to be popular choices in the rock world to add flavor.
I have come across another example, and I could use some help breaking it down. The progression (in B major) is:
I - iii - bVII - V
Now, I understand that a common technique is to dip into chords from the parallel or relative minor. In the case of B minor, the bVII chord is A dominant seventh, and it is the VII chord (is that redundant? I'm not used to talking about minor keys). In the case of the relative minor, g-sharp minor, the above chord is iidim – A sharp diminished. So, either I am missing something, or there are no clues there.
Another possibility I considered was the secondary dominant chord. In B major, the V chord is F# major. So, the V/V would be C sharp major. So, that can't be it.
Finally, because this chord is featured in a descending bassline that goes: B(I) A#(iii-inversion) A(bVII) G# G F#(V), I could understand viewing it as a passing chord thrown in sort of by feeling and instinct rather than arising from consideration of the functional purpose.
Regarding function, this chord does set up the stage for the V chord nicely. I am not sure if that is a harmonic relationship (if the bVII behaves like a sort of secondary dominant) or if it is just because of the flow of the descending bassline.
I would really appreciate anyone's input so that I can lay this to rest.