The 2nd flat in a minor chord progression

Question

If I have the following chord progression in a song in key of Bm:

Bm G A Bm

and then in the bridge a C major chord jumps in the progression, as below:

... D G A (C) Bm....

(C introduces the bridge and is followed by the tonic again)

How could be this 2nd-flat chord interpreted in terms of harmony?

According to my knowledges it:

1. can't be some secondary dominant for this scale (in Bm the F is #)

2. can't be a modal interchange (C is # also in Bmajor scale)

3. is not a translation inside the minor scale (it belongs neither to harmonic nor to melodic minor scales)

Answer 1

...interpreted in terms of harmony

I suppose you mean an interpretation according to common practice or functional harmony. But that isn't the only way to analyze harmony.

I think it might help to think of what's going on in mostly linear terms.

You have one step-wise line ascending to Bm in G A Bm.

If you regard some of the chords in the next part as an insertion/telescoping - I will put that portion in brackets - you have a step-wise descent to Bm in D [G A] C Bm where D C Bm would be the simple descent, but the G A is inserted and kind of refers to a portion of the preceding ascending line.

There is a nice juxtaposing of the C# in the ascent and the "lowered" C natural in the descent. The two tones are associated with linear direction.

Answer 2

We call this harmony, a major triad built on the lowered second scale degree, the Neapolitan. In the Classical style, it often functions as a predominant, moving to V and then to i (or I). But in other styles—like your example here—it can also function as a chord that moves directly to tonic.

And it actually can be understood as modal interchange, but not from minor: rather, it's actually borrowed from the Phrygian mode. B Phrygian is B C D E F♯ G A B, so the II chord in this key is C E G, that C-major triad. Because of this, we also call the Neapolitan the "Phrygian II."

Answer 3

There are various ways to hang a label in it. Neapolitan, Phrygian, ♭5 substitution (pity it wasn't a C7...)

Maybe the simplest way to legitimize it is this. You know you can always precede a chord with one rooted a 5th lower - remember that 'Cycle of 5ths diagram'? Well, add the 'rule' that a chord rooted a semitone above also works. Add it to your bag of tricks.