I’m a self-taught guitarist and teacher in Franklin, Massachusetts, and Mac’s Matrix is the thing I have been building, in one form or another, for about fifteen years.

It started with a piece of wood. I drew a fretboard on it and cut transparent sheets to lay over the top, each one dotted with a scale. I wasn’t teaching then — I was trying to show myself something I could feel but not explain: that the fretboard never really ends. Slide a shape up twelve frets and it comes back. Every pattern repeats, because the intervals underneath it do.

The overlays showed that. They couldn’t explain it. Three years ago I worked out the grid that does — twelve columns, one for each interval, read as a two-octave frame. It is the same frame that makes a guitar repeat every twelve frets and a piano repeat every twelve keys, which is why one diagram can serve both instruments, and any scale built from those twelve semitones.

In March 2026 I started writing the app, to put the theory, the chord shapes and the scale patterns for all twelve keys in one place, organized by interval rather than by notation — so the relationships are identifiable rather than esoteric.

Three decades of playing sits underneath all of it. The Matrix is the diagram I wish someone had handed me at the start, and it is the one I hand a student now.

Jeff Mac’s Music

Working musician, not a retired one — that’s the whole reason the teaching holds up. The playing and the teaching aren’t separate things: what I show a student on a Tuesday is usually something I had to solve on a stage.

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