Excerpt from The Theory of Harmony: An Inquiry Into the Natural Principles of Harmony, With an Examination of the Chief Systems of Harmony From Rameau to the Present Day English theorists: Day's Treatise on Harmony (184 5) Diatonic and Chromatic Harmony The Key-system Natural Discords Day's Fundamental Bass: System of Roots Minor Harmony and Minor Mode: The Subdominant: The Augmented Triad.