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Speaking of HTG. My mind was recently blown by the idea that the first chord of the bridge of Ipanema is a IV chord in the key of Db. I’d always thought of it as a I chord. In LCC terms this new way (Db) makes the horizontal focus for the first two chords of the bridge Db major. Since then I’ve been experimenting with changing the first chord to Dbmaj7+5 or going Dbmaj7-G7#11, Gbmaj7. I haven’t exhausted all the possibilities( like sneaking in a little Db blues over this section).

That bridge seems one of the most interesting of that period - early 60's - to me. People have always asked questions about it. I agree, it might be Horizontal, since the second chord can be seen as the very common IV min 6th in Db. The rest of the chords are equally intriguing; the whole tune has a "flat 5" quality to me.