In the C, Ami, F, & G7 progression (or I, vi, IV, V7 and back to I) a standard for the 1950's but used in 1938 for "Blue Moon." There is a progression I use Cmaj7, C#dim6, Fmaj7, G/b9, Cmaj7. On guitar, the bass notes are C, C#, A, G#, and C. Why does the G# of the G/b9 chord fit in so well? The G chord (V) wanting a G in the bass note at that point is so dominant, you would think that your ear would not accept a G# there. chords used

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    That G# is in fact an Ab. Otherwise it wouldn't be called G7b9.
    – Tim
    Jul 24, 2022 at 17:32
  • Alright why does the Ab work when the G is so donimant in the progression?
    – Fred
    Jul 24, 2022 at 17:34
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    IMO, a G/Ab sounds weird, if it's the lowest possible Ab on a guitar, like 4x0003. It doesn't work for me. What is the exact voicing you're using? Higher up it's good, like xx6433, great chord. Or a "flamenco dominant" 356433 which is something like "G (add b9)". The xx6433 could be called an inversion of "G (add b9)" or maybe "Ab dim maj7". :) Never seen that written though. xx6533 is nice as well. Jul 24, 2022 at 18:35
  • C#dim6 = C#-E-G-A# but C#-G-A#-E is the particular inversion I am using.
    – Fred
    Jul 24, 2022 at 18:44
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    I solved the whole mystery. It's a minor plagal cadence in C, the OP confused the Fm with G because the F - Fm - C progression is such a strong resolution to the tonic C. Jul 24, 2022 at 20:29

2 Answers 2


From your comment:

Yes the Ab is the lowest Ab on the guitar. The voicing I am using for the G/b9 is G (3rd fret on first string), E (5th fret on second string), C ( 5th fret on third string), E (3rd fret on fourth string) and Ab (4th fret on the sixth string).

So the actual chord you're playing is: 4x3553. The 3rd fret on the fourth string is an F, not an E. There is no D or B of any kind anywhere in the chord, so it's not a G major.

I would call it "Fm maj9 / Ab". Maj9 means that there is a major seventh (E, fret 5 on the second string), and a ninth (G, fret 3 on the first string) added to a basic F minor triad.

Fm maj9 /Ab

The Ab bass note has been moved from the stack of thirds an octave lower. Otherwise it would be a perfect stack of thirds.

Fm maj9 /Ab inversion explained

To simplify it in terms of basic triads, you could play a plain F minor chord instead. With Fm/Ab you get the same bass as well. Just play the lowest three notes from the chord diagram above, that's an Fm/Ab, or first-inversion F minor triad.

Why does it work? An Fm chord is often used in songs in C major. You could say there's "borrowing" or "modal interchange" or "chromatic alteration" going on with the A being flatted to Ab. And I would say that B is flatted to Bb as well, even though none is played - try it out, would a melody Bb or B work better during that chord.

To simplify the whole chord progression to bare minimum plain major and minor triads with no frills and niceties, it would go like this: C - A - F - Fm.

Now the question, why did you identify the Fm chord as being a G major? It's because in an F - Fm - C progression, the Fm works nicely as a final chord before going back home to C. There's even a name for this: a minor plagal cadence.


A minor plagal cadence, also known as a perfect plagal cadence, uses the minor iv instead of a major IV. With a very similar voice leading to a perfect cadence, the minor plagal cadence is a strong resolution to the tonic.

The validity of that name and whether the quoted text even makes sense, is disputed, as seen in the comments. I don't particularly care about what it's called on theory courses around the world, and outside this forum, I have no need to categorize cadences. But iv-I is a thing, and a good resolution to I.

So maybe because you're used to G being an ending chord that leads back to C, you thought this must be a G, because it leads to C so well.

  • Thanks. This delima has bugged me for a long long time. I like your explanation.
    – Fred
    Jul 24, 2022 at 21:39
  • @Fred Did you try the simplified version C - A - F - Fm, doesn't it convey the essence of the progression? Jul 24, 2022 at 21:52
  • Yes I tried it and it does convey the essence of what I hear with the fuller chord. It is a progression that I recognize but didn't think of it being applied here - I guess I got hung up on the V to I resolution. Since you pointed it out it makes perfectly good sense. Thanks again.
    – Fred
    Jul 24, 2022 at 22:19
  • If you think that this answer solves your question, and no further possibly better answers are being added for a couple of days, please mark it as the accepted answer. And if you could move your fretboard illustrations into the actual question, then the question would be more understandable. Click Edit to edit your question text. Jul 25, 2022 at 0:02
  • That Wikipedia quote is nonsense. First it says that "minor plagal cadence" and "perfect plagal cadence" are two names for the same thing, and then it says that they have similar voice leading, which implies that they are different things.
    – phoog
    Jul 25, 2022 at 6:38

I suspect C, C♯, A, A♭, C are not really a bass line, just the bottom notes of your guitar chord voicings. This sort of thing is not uncommon in guitar stylings. It 'works' (to a degree) because G(♭9) is a standard embellishment of G, and, in the context of chordal guitar playing we tolerate strange inversions. Though I also suspect it might not bear scrutiny QUITE as well as @Fred believes! Could we see notation of exactly what he's playing, or hear a recording?

  • There are analogous left-hand piano voicings... In the end, I think they are not truly described just by a chord symbol... but at the very least by chord symbol plus bottom-note specification, as a hint at the desired voicing. In the present example, I'd believe that the function of the chord was vaguely G7-ish, but that needn't completely specify it. The G7flat9 seems to me more of a rationalization than explanation. Just saying it's a voicing of a diminished chord may be more to the point. Jul 24, 2022 at 19:56
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    @paulgarrett The OP listed all the notes, there's no B or D of any kind in the chord, so it's not a G major. The OP doesn't know what he's talking about, which is usual for many posters on this forum - that's why they post and ask questions. A lot of guesswork and approximation is needed to decipher the actual problem behind the text. Jul 24, 2022 at 20:12
  • @piiperiReinstateMonica, ah, indeed. :) Jul 24, 2022 at 22:54
  • @piiperiReinstateMonica It is odd, the chord is named as a G/b9, which sounds to me like the correct chord in that song, but written as a C/b6. That's just a "dumbing down" of the chord progression, returning to the C chord a bar early and using the flat 6 to lead into it. I don't know where the OP got these chord voicings from but it looks like kind of a cheat; I'd recommend finding a different voicing that is actually G/b9. Jul 25, 2022 at 14:12
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    @DarrenRinger The OP's name for the chord was wrong, he was calling "G" something that was not a G chord at all. The reason for calling it a G was, the chord provided a nice resolution to C, so the OP thought it must be some kind of a G chord. But G is not the only way to head home to C, you could use, say, Fm or Db9. The dots in the OP's diagrams are what he's actually pressing down on the fretboard, but some of his names for the chords and notes are a bit off or entirely wrong. Jul 25, 2022 at 14:16

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