Good news — his gut instinct (“B flat major sounds better”) is actually the right answer. Here’s why, because the mix-up is a genuinely confusing notation quirk:
Two different chords are both sort of “attached” to the number 7, and they’re not related to each other:
| Root note | Chord | |
|---|---|---|
| vii° (the one from the diatonic table) | B — the natural, unaltered 7th note of C major | B diminished (B–D–F) |
| bVII (the borrowed chord) | Bb — the 7th note flattened down a half step | Bb major (Bb–D–F) |
The key thing to clear up: the “b” in “bVII” doesn’t mean “take the vii° chord and make it flat.” It means something more specific: first lower the scale degree’s note itself by a half step, then build a brand-new major chord from scratch on that lowered note. It’s not a modified version of the B diminished chord — it’s a totally separate chord that just happens to also be “based on scale degree 7.”
So worked out in his key (C major):
- Scale degree 7 = B
- Flatten it a half step → Bb
- Build a major chord on Bb → Bb major (Bb–D–F)
That also matches the plain-English description in the lesson — “a major chord a whole step below the key” — C down a whole step is Bb, so it checks out.
The one-line rule to give him: whenever he sees a flat sign directly in front of a roman numeral (bVII, bIII, bVI, etc.), it means “flatten that scale-degree note first, then stack a major chord on it” — completely separate from whatever chord already exists at that scale degree in his diatonic table.