What is the theory behind this chord progression with two chords from the minor scale of the same key?

Question

I came across a lovely piece with the following chrod progression:

Body: D Dmaj7 G Em F#m G Em D
Ending: D D7 Bb C D D

From the body it looks like D major scale, but in the ending two chords from D minor scale are added before proceeding to the D major chord, like a picardy third, altohugh the piece is not in D minor.

My question is how does the ending work in theory? Like, why does D-D7-Bb sound smooth? Is it a picardy third? (also, how do we notate it by roman numbers?) Are there other examples using the same trick?

Thanks :)

Poss. dupe for 'is there a name for this cadence?' – Tim Jul 10 '23 at 18:18 — Tim, Jul 10 '23 at 18:18
@Tim You are right! I just realized that and flagged it :) – EconomicsSucks Jul 10 '23 at 18:59 — EconomicsSucks, Jul 10 '23 at 18:59

score 3 · Accepted Answer · answered Jul 10 '23 at 15:13

3

It's modal interchange. Bb is bVI and C is bVII, both borrowed from D minor.

Maybe one could think of D7 as of V/iv and then bVI substituting iv?

See also: Is there a name for this cadence? (bVI bVII I)

answered Jul 10 '23 at 15:13

user1079505

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What is the theory behind this chord progression with two chords from the minor scale of the same key?

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