One easy question. Why “biv” and not simply “iii” chord? Is that because you’re taking that “biv” chord from a scale where the IV grade is disminished?
With IV grade I mean the “IV interval” in the scale
iv♭ in the context of C major would be an F minor chord. The iv referring to the root, and the ♭ referring to the third. Anytime you see an accidental to the right of a roman numeral and it is alone, it refers to third of the chord. If it had been to the left, it would have been an F♭ chord. While that sound strange, instead of saying E, there are cases in which enharmonic spellings make more sense, usually in connection with modulation. For instance, if you wanted to modulate to C♭ major, F♭ would be a more appropriate chord choice than E major.
The flat sign may be superfluous but I preferred to be a little more explicit. Because it is in relation to a major I chord, I felt adding a clear flat 3rd would help understanding. I may update it to add example chord symbols though.
Is the flat sign after the iv really necessary? The iv indicates a minor chord and in the key of C Major it is F minor and the 3rd in the F minor chord is an A flat.
Perfect, now I get it. Thanks Jon!