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I couldn't quite understand the movement of this progression and the function of both Abm7 and Am7-5 chord in this progression.

Fm || Abm7 || Cm | Bb | Am7-5

Do they move to minor scale momentarily through Abm7 and Am7-5? or is there another reason for this movement?

it's from a japanese song

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  • Where do you have these chords from? I can’t here this progression. Sep 12 '20 at 21:27
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The passage quoted is a prolonged predominant.

We're in the key of Eb major, of which the I chord immediately precedes the chords in question.

ii (Fm), iv7 (Abm7) (borrowed from Eb minor), and vi (Cm) all serve as predominant chords, easily moved between because of their shared tones.

From Cm the remaining chords arise from a chromatically descending bass line, which leads to Ab (IV, another predominant), the chord immediately following the OP passage.

Illustration

ii, IV, and vi can be stacked up as a single chord: ii9

X:0
T:ii, IV, and vi chords, stacked
K:Eb
M:none
L:1/1
[FAc] [Ace] [ceg] || [FAceg]

Incorporating the iv chord from minor (i.e., replacing C with Cb) effectively creates a half-diminished nine chord.

X:0
T:ii, iv, and vi chords, stacked
K:Eb
M:none
L:1/1
[FA_c] [A_ce] [_ceg] || [FA_ceg]

The bass movement is what creates the remaining chord progression.

X:0
T:chromatic descending bass
K:Eb
M:none
L:1/4
%%score (V1 V2)
[V:V1] [eg] [eg] [ceg] [ce]
[V:V2] "_Cm"c "_Eb/Bb"B "_Am7b5"=A "_Ab"_A

In a hyper-local interpretation, the Am7b5 could be viewed as Bbbm7b5, which would be biimb5 relative to Ab.

Abstraction

X:0
T:Abstract harmonic interpretation
K:Eb
M:none
L:1/1
%%score (V1 V2 V3 V4)
[V:V1] [| "_Eb""Eb"[EGB] || "_(Fm9""Fm"[FAc] || "_Fmb9b5""Abm7b5"[A_ce_g] || "_Fm9)""Cm"[ceg]/2 |  "_desc. bass""Cm"[ceg]/2 y y || "_Ab""Ab"[Ace] |]
[V:V2] x x x x/2 =c/8x"Eb/Bb"B/8x"Am7b5"=A/8
[V:V3] x "<("">)"[eg] "<("">)"F
[V:V4] x x x "<("">)"[FA]/2
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My amateur speculation with lots of secondary dominants:

  • Fm Abm7 Cm might be iv-V-i in C minor (Abm7 is similar to G7)
  • I think Bb should be Eb (inversion), I don't hear a D natural in the chord
  • Cm Eb Am7b5 might be ii-V in Bb (Am7b5 is similar to F7, and you can think of the Bb note in the Eb chord as just a passing tone from the C in Cm to the A in F7)
  • So overall, it is iv-V-i in C minor, then the unresolved ii-V in Bb, then IV-V-I in the subsequent Ab Bb Eb.
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a very common progression: Yesterday (Beatles)

I (ii7b5 V7)/vi

The song is in Eb.

Do you mean the passage I - ii - iv?

Eb Fm Abm (Abm = minor subdominant)

I don’t think that there’s really an a minor chord in it.

This could be a chromatic passing chord V- bV - IV:

(Bb - Bbb - Ab)

Now I’ve found may be a quite correct chord sheet where you can see after the dominant Bb it goes back to the relative vi (cm) where the bass plays this vibrato on the chromatic tone a or Bbb which has an ambivalence beeing the root of the secondary dim7 of Bb (= a) or passing tone to Ab (B double flat)

Chords in C major (Jap. lyrics)

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It's not a move to minor, just temporarily borrowing the G-Flat. The whole sequence functions as predominant. In order the chord sequence functionality is:

  1. The first chord Fm11 (diatonic minor ii-11)
  2. The second chord Abm9 is a borrowed iv-9 from the parallel minor.
  3. Move to vi (Cm) is a bit unexpected, but it continues the pattern of jumping up a third in the bass, while other notes either hold in place or shift by a half step. Resolution to I or V would be more common immediately following the previous two chords.
  4. Bb in the bass is not the root, it's a I-64 formed by a bass passing tone
  5. Bass drops to A-Natural, now forming half-diminished, which is still a predominant. (it's vii°-7 of V)
  6. Ab major chord, diatonic IV

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