Rather than derail the OP's mode thread - I thought I'd start anew - the mode thing will come up for sure but - personally I'm not going to bother with modes at all I have talked melody,voice leading, key areas and tonicisation. But that doesn’t mean someone else won’t go there. Talking modally doesnt exclude chromaticism either – I’m just going to talk melody now. Im going to talk about the tones targeted by the melody as being intrinsic to the way this tune voice-leads. So when I mention pairs of notes Im actually referring to the main points in the melody -not every melody note. Look at the melodic gravity in the first 8 bars first – it might tell you something about modality in this tune. This tune differs from many other standards in that the A section resolves repeatedly to the fifth , not the tonic. In fact if you follow the melodic framework in the tune - it basically heads from G to C - which in effect creates motion to a tonality of “C” Unfortunately I cant seem to get this thing to upload properly - but at the bottom is the melodic outline of the A section - above it (should be below it- is the B section) I don’t buy the simplification that this is just 9th to 5th voice leading (although it is. ) (The gravity from the 9th continues in the B section too). Jobim has set us up with a melody that outlines a C tonality against a harmony in F tonality. As a soloist I always acknowledge this even at a subconscious level on this tune i.e – the tonic has an implicit C/F sound to it because of the melody.The other paradox is that the consonance in the melody is stronger against the G7 than the Fmaj - cool. The melody doesn’t resolve to F until the B section which immediately places the tonic note in tension with Gb. Jobim was such a mofo composer! In the B section there is some really interesting stuff happening - Jobim clouds the tonality even more by finally stating the tonic note F but in context of Gb major – so it has a certain tension. This tension drops to the 3rd of Cb7 (Eb) The same thing happens up a tone (Ab to Gb). Then up a semitone – (A to G ) The harmony here is also a disguised sequence. Gbmaj Cb7 F#m9 D9 Gm9 Eb9. (On the other hand - knowing how Jobim thought of chords on the guitar – there is another way of looking at this which is altogether simpler - more on that later.) The sequence is obvious in the last pair of chords - m9 down a maj 3rd to a dominant chord. Not so obvious in the first pair ( Gbmaj –B7) until you realise that Jobim has subbed the Gb maj 7 for what could also be an Eb min9 chord. ( Ebmin9-B7) the Major 3rd motion. The shift to a C in the melody starts a iii Vi ii V progression back to the A section - but targets melody tones in maj 3rds - C – Ab then Bb –Gb - the same motion as the chords in the Bridge and then ends with the Gb - so the whole bridge does this long journey from the Tonic F to Gb - which is of course destined to resolve back to the G in the melody. Another subtle hint at another tonality. I have more to say about how to approach the chords as an improviser– especially how thinking modally in the bridge might actually help one to think more globally! - but someone else can have a go - I just wanted to emphasise the importance of the tune itself!