It's really an augmented sixth chord (D-F#-Ab-C) in both occurences in the
tune. It appears again in m.21 and again resolves to Bb. There are a number
of ways to look at it.
Traditionally this type of progression probably began as IV - iv - I
(Eb - Ebm - Bb) as a plagal cadence. Somewhere along the way, a bass player
who liked to keep the line moving down in 5ths probably played an Ab.
With Ab in the bass, it now sounds like an Ab9 chord. An Ab9 chord may
function as a tritone sub dominant to Gm, which is the relative minor to
Bb. so in a way, the Ab9 to Bb is a deceptive resolution. A typical
deceptive resolution is when V7 resolves to vi, the relative minor. This
case is the flip side, when a dominant pointing to the relative minor
actually resolves to major.
I have seen others explain the Ab7 as the V of Db. It's not possible with
the melodic and harmonic implications. In m21 the melody note is D natural
and sounds like a perfectly natural note (the tune is i Bb major).
Compare the three measures: |Ebmaj7///|Ab9///|Bbma7///|
Eb - F - G - Ab - Bb - C - D - Eb for Ebmaj7
|
Eb - F - Gb - Ab - Bb - C - D - Eb for Ab9. One note needs to change: G to Gb
| |
Eb - F - G - A - Bb - C - D - Eb for Bb (shown Eb - Eb for ease in comparison)
the A & G are now natural.
Several approaches when confronted with that information:
1. Play emphasizing the numerous pitches that are common to each scale
2. Call attention to the pitches that change
3. Combination of the above
4. using what you know of the pitches above and what you know of chromatic
embellishment
Some call the Ab9 chord in that progression a BACKDOOR DOMINANT. The scale
that accompanies that sound is a mode of Eb melodic minor, often called Ab
lydian dominant because the first tetra-chord is lydian and the second is
dominant:
Ab-Bb-C-D (lydian) Eb-F-Gb-Ab (dominant)
This kind of progression occurs frequently or I wouldn't have invested this
much in a reply. Other tunes where you may find the IV moving to the
"backdoor dominant" resolving to tonic include:
Misty
There will Never be Another You
Cherokee
_______________________________________
Bert Ligon
Director of Jazz Studies
_______________________________________
School of Music
University of South Carolina
Columbia, SC 29208
Voice: (803) 777-6565
Fax: (803) 777-2151
http://www.music.sc.edu/Departments/Jazz/
bligon@mozart.sc.edu
_______________________________________