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I don't know much about music theory so I need some explanation!

While playing a tune, I came across this:
(in F minor) Bbm / Bdim7 / C7 / Fmin

Where is that Bdim7 coming from? Would I call it a bV° or something else? And why is this working in that context (the Bdim7 being the vii° of C, I imagine that's why it lead well into it here).

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Welcome to Music.SE!

This is what we call an applied chord (some also call it a secondary chord). And your guess is exactly correct: this is a chord that functions to briefly make another chord sound like tonic.

This B°7 is the °7 chord built on the leading tone (scale-degree 7) of C major. Since this B°7 exists in C and not in F, playing it makes the music briefly sound like it's in the key of C. We call this act tonicization. You may have heard of modulation—a complete, lengthy move to another key—but a tonicization is a more brief move to another key. Think of modulation as jumping into the pool, but tonicization is just dipping your toe in. (See also What's the exact difference between modulation, key change and tonicisation?)

Since this B°7 chord is functioning to make C seem like a temporary tonic, we want to label the chord to show that function. As such, instead of labeling this ♭V°7, it's better understood as vii°7/V, showing that it is the vii°7 chord of V (C) in the overall key of F.

As such, your entire progression is best understood as i–iv–vii°7/V–V7–i.

While this chord is called a secondary leading-tone chord, there are other types of secondary (or applied) chords. The secondary dominant is probably the most common, and that's where you use the temporary tonic's V(7). Trying replacing that B°7 with a G7 to hear what a V7/V sounds like!

  • I knew about secondary dominants, but not of those other types of applied chords. I need to study all that, sound fun and interesting! thank you for your answer, I think the concept and use of that °7 herre is clear in my mind now! – bmoineau Sep 8 '18 at 22:16

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