Lush Life: Understanding the Harmony Structure behind the music of Billy Strayhorn PART ONE

There have been a few Tutorials on how to play Billy Strayhorn’s Lush Life. However; as far as I know, there has not been a Tutorial explaining the theory behind the music. No doubt about it, the music is complex but well worth the effort and time spent to understand it. Welcome to PART ONE of two parts. If you can understand what is going on here, you will be in a position to apply some of these considerations to other songs.