Thelonious is a music theory app designed to help you understand keys, chords, scales, and harmony through clear, visual layouts. Explore major keys, review scale notes, and study diatonic chords in a format built for both learning and quick reference. Thelonious connects theory to real playing with chord progressions, guitar mode charts, and CAGED system material. Use Thelonious to: • Explore major keys and their scale notes • Understand diatonic chords and key relationships • Learn relative minor and dominant chords • Study chord progressions and harmonic movement • View guitar modes and CAGED chord charts Whether you’re learning the fundamentals or deepening your understanding of harmony, Thelonious makes music theory easier to read, understand, and apply.