Plectrum Theory

About this app

Music theory for guitarists, taught on the fretboard. Most music theory is written with the piano in mind, leaving guitarists to translate every concept to the neck. Plectrum Theory starts with the guitar. Every idea is shown as something you can see, hear, and play, directly on the fretboard—so you understand how it connects, not just what it is. THE CURRICULUM Seventy-three lessons across thirteen modules, arranged to build naturally from the fundamentals to improvisation. Foundations — notes on the fretboard, intervals, and the building blocks of music Scales & Keys — scale construction, key signatures, and the modes Chords — triads, open chords, the CAGED system, and seventh chords Harmony — diatonic harmony and chord progressions Improvisation — applying theory to create music Lessons are short and focused. Each ends with a review question to reinforce what you've learned, while progress tracking and bookmarks make it easy to stop and pick up where you left off. THE REFERENCE A practical reference you'll keep returning to long after you've finished the lessons. Intervals — every interval with formulas and fretboard diagrams Scales — construction, formulas, and fretboard positions Chords — nineteen chord qualities from triads to thirteenths, each with formulas and diagrams CAGED — the five interconnected shapes across the neck Progressions — the essential progressions and the harmony behind them Fretboard Notes — every note across the neck in every key BUILT FOR LEARNING Every diagram is drawn on a real guitar fretboard. Interval labels appear on every shape, making patterns understandable instead of something to memorize. Portrait and landscape layouts. A distraction-free dark interface designed for long study sessions. Works entirely offline.

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1. First observed Aug 21, 2026 · 7:18 AM $9.99