Why does this IV-iii-i6-ii chord progression work?

Question

I've been listening to a lot of jpop and I've heard a lot of this chord progression, which is basically IV-iii-i6-ii.

In the song that I've linked below, I think it's in G, so the progression is basically C-Bm-Gm/Bb-Am.

I've heard a lot of other variations of the chord, sometimes it's a diminished chord and other times it's a major chord, so my ears might have heard wrong but I think it's a Gm/Bb.

I know the progression works basically because it's chromatic, but how does a minor i chord function within a key to make this particular song work? And how does a diminished version of that chord function? (in this key a#dim)

Answer 1

I'll simplify it, removing some embellishments and leaving at most 2 chords per bar: (starting from 1:26)

| C | G/B | Am B7 | Em G7 |

| C | G/B Em | Dm7 G7 | C Am |

| C D | G F/G | C B7 | Em Dm7 |

| C D | G A7 | C | Am D |

Then the questions:

• Q: Why is there a Dm7?
• A: it works like a complicated G7 chord. Dm7 - G7 - C is a longer way to say G7 - C.

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• Q: But there's a Dm7 - G7, not just G7.
• A: Dm7 - G7 - C does the same thing as G7 - C, just with more chords.

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• Q: Why is there a F/G?
• A: It's a different way of saying "G7".

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• Q: What's the A#dim or something before the Am?
• A: It's an in-between stepping chord that has many meanings. You don't need to "understand" it - you can insert an A#dim7 when going to an Am. Or if you insist, you can "understand" it by seeing it as a tritone substitution or chromatic chord or anything, all explanations are equally valid. IMO the best explanation is "I insert this dim chord half-step above the target". You could use an A#m7 just as well and it would do pretty much the same thing.

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• Q: But there's a D# bass in the B7.
• A: Yes there is, it's an inversion. You can take any chord and move the bass to any of the chord tones and it will do the same thing, just a little bit differently. If you play the same chord for the whole bar but you want to make it sound like there are two chords in the bar, do an inversion in the middle of the bar. Or any kind of bass movement, or actually you could move any voice anywhere, as long as it doesn't completely ruin the function of the chord.

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• Q: But there are more notes than plain C, D, Am etc. "add9", "maj7" ... all sorts of extra notes?
• A: They're filling and flavor enhancers. Try adding all sorts of notes to all chords and develop a taste for what you can add where. For example you could replace a D chord if it works as a dominant heading to G, with C/D.