This is the definitive study of chord progressions of hundreds of carefully chosen tunes from the jazz musician's repertoire, comparing them, linking them together by commonalities, and codifying harmonic traits that will clarify the reader's understanding of how progressions work. It not only provides the means to learn and memorize tunes more easily, but also becomes the key to acquiring the skill to cognize progressions by ear! There is more. The Preface explains how we hear and understand music, establishes the methods by which we can learn to develop our musical hearing more efficiently, and addresses the importance of attitudes, as they affect the learning process. The Afterword offers further encouragement, suggests ways to continue to study, and ways to personalize the materials beyond what is presented here. Appendix A reveals the valuable list of nearly 500 tunes used in this study, indicating which tunes contain more than one harmonic trait, which facilitates more efficient choices of tunes to learn. Appendix B renders valuable, ear-opening tips for ear training. And Appendix C illustrates road maps for ten tunes, showing how progressions can be more easily understood, memorized, played, and transposed to any key!