This diagram is called the "Music Theory Tree." It is a very comprehensive and well-designed visual tool for understanding music theory in the key of C major. What it represents: It is a circular representation (almost like an astrological wheel or a mandala) that brings together several fundamental, interconnected concepts in one place: The C major scale (C D E F G A B) at the center. The 7 diatonic chords (triads) of this scale: I: C major (CM) ii: D minor (Dm) iii: E minor (Em) IV: F major (FM) V: G major (GM) vi: A minor (Am) vii: B diminished (Bdim) The 7 Greek modes (or relative modes) of the major scale: Ionian (Ionian) → on C Dorian → on D Phrygian (Phrygian) → on E Lydian (Lydian) → on F Mixolydian (Mixolydian) → on G Aeolian (Aeolian / natural minor) → on A Locrian → on B The degrees (Roman numerals) and their harmonic functions. The notes, intervals, chord formulas, and colors to better visualize the relationships. The large red "C" in the center symbolizes the tonic (the reference note of the key). What is it used for? It's a very useful educational and practical tool for: Instantly visualizing all the relationships within a key (which normally takes several pages of lessons). Composing: knowing which chord to use in which part of a piece. Improvising: knowing which mode to play over which chord (e.g., playing the Dorian mode over a Dm, the Mixolydian mode over a G7, etc.). Understanding music: seeing how everything is connected (scale → chords → modes). Easily transposing to other keys once you've mastered the C major model. Learning the modes and their respective sonic "colors." In short: It's like a road map of the key of C major. Instead of learning separate lists (scale, chords, modes), you have everything in one interconnected diagram. It is particularly appreciated by guitarists, pianists, producers and composers who want to have a quick overview.