Im6 chord in a minor II-V-I
In a minor II-V-I progression, we usually have a IIm7(b5) chord followed by a V7(b9,b13) chord, resolving into Im7. I always thought the diminished 5th in the II chord (also the minor 9th in the V7 chord, same note) was like a "hint" that a minor chord was coming, since it corresponds to the minor 6th of the I chord. That would be F in A minor/C major.
If you analyze the progression from a relative major standpoint:
VIIm7(b5) _ V7/VI(b9,b13) -> VIm7
In a major harmony, the VIm7 is related to the Aeolian mode and should have a minor 6th, right?
However, it came to my attention that some standards, like Autumn Leaves and Caravan (no II here, but a long time spent on V7b9), have this chord written and played as Im6 sometimes, with a major 6th instead. Is there a particular reason for this?
Edit: Is this a chord substitution? Some substitutions are based in harmonic concepts, like the tritone substitution. Does this major 6th thing have such an underlying concept?
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You are right that in minor the minor sixth of the key is part of the II chord, and it is a tension (b9) that is very often used on the V chord. However, it is not a valid tension on the I chord, because the minor sixth is an avoid note on that chord (basically because it is a half step above a basic chord tone, the 5). In this blog post you can read more about avoid notes.
On the other hand, the major sixth is a valid tension on the I chord in minor. A minor 6 chord always contains the major sixth. It comes from the melodic minor scale. The I chord in minor comes in three basic flavors: Im6 (with a major 6th), Im7 (with a minor 7th), and Im maj7 (with a major 7th). The corresponding chord scales are melodic minor (for Im6 and Im maj7), and dorian (for Im7).
I think the answer to your overarching question is: there's no requirement that the ii chord or the V chord share the same scale/chord tones as the i chord. The ii-V-i progression will still sound good even whether or not a common scale is used across all three chords.
For example, it's not uncommon to play a Locrian ?2 scale over the ii chord, an altered scale over the V chord, and then a melodic minor scale over the i chord. If you take a ii-V-i in G min, this would be equivalent to playing these chords and scales, respectively:
| Aĝ7 | D7alt | Gmin | Gmin |
| C melodic minor | E? melodic minor | G melodic minor | G melodic minor |
There are plenty of notes that aren't shared across these three scales, but those scales nonetheless sound good over the ii-V-i.
That said, there might be reasons for choosing to play Aeolian vs. Dorian vs. melodic minor over the i chord. Matt L. mentions one possible reason for choosing dorian over Aeolian--because Dorian has fewer avoid tones. On the other hand, one might choose Aeolian for continuity with the V alt chord, as you suggest in your original post. Another factor one could consider when choosing a scale is the melody--if the melody uses a ?6th over the i min chord, then maybe the solos should too. But there is no prescribed scale and the soloist often has freedom to choose which to use. So the Aeolian can be a great sound even despite its ?6 avoid tone, but there's no rule that Aeolian is required over the i min chord to create continuity with the iiĝ or V7alt chords.
A couple notes:
A i min6 chord is extremely common on Autumn Leaves, in large part thanks to Miles Davis, who in bar 7 plays the natural 6th in the melody instead of the root. (This link starts after the intro, and the natural 6th is heard at 1:06.)
Playing i min6 instead of i min7 doesn't qualify as a substitution because we're playing the same chord, just with different alterations.
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