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I'm looking through a piece of music that has two flats in the key signature. It feels like it's in D minor (the last chord), and the verse alternates between E♭9 and Dm6.
A couple of things - why would the arranger use that key signature? Would the E♭9 be a tritone substitute for what could easily be A7? Could it actually be in a mode instead?
It's Dizzie Gillespie's Night in Tunisia. I've played it with one flat, which somehow makes more sense than 2.
Without judging whether it appropriate, I think the reason someone might use two flats in the key signature is the Eb7 chord of the vamp and the E flat at the end of the first part of the melody...
...could be considered to come from the Phrygian mode rather than tritone substitution/altered dominant harmony.
If a tritone substitute is considered clearly identifiable only when part of a descending fifth progression, then it seems reasonable to say the Eb7 chord here is not a tritone substitution, but a chord rooted on the lowered second degree of Phrygian.
In other words iim7b5 bII7 i would be a clear case of tritone substitution, bII subs for V, because it substitutes in a progression of three roots by descending fifth. But just alternating bII7 i bII7 i is not the clear substitution within a descending fifths progression pattern.
So, if someone took that view - the Eb7 is not a tritone substitution - the chord is diatonic to Phrygian (at least the base triad), the Phrygian key signature reinforces that idea.
When the tune gets to using a dominant rooted on A at end the phrase, and it has a proper leading tone, that doesn't contradict Phrygian, not in the classical sense, so that seems like additional support for Phrygian.
Either way it doesn't change what's happening tonally. It's a little like when Baroque scores would use zero flats for D minor. The sharps and flats in the score make the final determination of tonality.
I think the E♭9 is the "foreign" chord here. So i think the piece (hard without having seen the whole piece) is correctly in D minor, with the E♭9 possible a tritone substitute.
I think you were on the right track, but if you can share the whole piece, then i can get it right 100%.
