So I came across a song in which the chord progression consists mainly of 7 chords, but they don't seem to be secondary dominants.
It sounds very jazzy and I am aware that jazz compositions sometimes contain all these 7 chords, but I have a hard time analyzing it from a tonal point of view.
Here is the progression :
Cm7 - Eb7 - D7 - Ab7 - G7
I know it might sound very basic, but I really couldn't come up with something more precise for the moment.
The E♭7 and the A♭7 are both tritone substitutions which wile not exactly being secondary dominants very much take the role of them tonally and are often analysed as such. The E♭7 will want to take you to D7 and the A♭7 will want to take you to G7 just like the D7.
I've seen tritone substitutions marked before as "tt" so if doing Roman Numeral analysis you may see this:
X:1
L:1/4
M:
K:C Minor
V:1 clef=treble
"i7(Cm7)"[C E G B] "tt(Eb7)"[E G B _d] "(V7/V)D7"[D ^F A c] "tt(Ab7)"[A c e _g] | "V7(G7)"4[G B d f]||
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More formally and what I would do is note the proper function of the tritone substitution like this:
X:1
L:1/4
M:
K:C Minor
V:1 clef=treble
"i7(Cm7)"[C E G B] "V7/ii(Eb7)"[E G B _d] "V7/V(D7)"[D ^F A c] "V7/V(Ab7)"[A c e _g] | "V7(G7)"4[G B d f]||
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Notice how both the D7 and A♭7 have the same exact function in this analysis which is very much intentional as the serve almost the exact same function.
Another analysis of Cm7 - Eb7 - D7 - Ab7 - G7 would be:
i - G7/V/V - V7/V - G7 - V7 or a German Sixth of V7/V then V7/V followed by the German Sixth (in C-minor) then V7.
It's sort of a i-V7 sequence with first a Secondary Dominant inserted i-V7/V-V7 then each of these is preceded by a German Sixth. This not the only analysis but it does show that the pattern isn't out of place in Common Practice Period Harmony. The functional relationship between the Eb7 - D7 (and the Ab7 - G7) is different from the more jazz-oriented analysis in Dom's response. Here these two are Subdominant-Dominant (or Predominant-Dominant according some authors) rather than Dominant-Tonic relations. Of course, these two sound the same (but may accompany different style melodies.)
