Before the shapes, before the scales, there is one question worth being able to answer instantly: where is this note? Every shape in the CAGED system is anchored to a root, and if you cannot find the root the shape has nothing to hang on.
The lattice
Strip the neck back to a single note and a pattern appears. In C there are eight of them inside the first fifteen frets, and they are not scattered: they sit in a lattice you can walk.
The octave shapes
The lines in that diagram are the ones every guitarist ends up knowing without being taught. Each joins a note to the same note somewhere else, and there are only really four of them.
| From | To | Move |
|---|---|---|
| 6th string | 4th string | two strings over, two frets up |
| 5th string | 3rd string | two strings over, two frets up |
| 4th string | 2nd string | two strings over, three frets up |
| 3rd string | 1st string | two strings over, three frets up |
| 6th string | 1st string | straight across, same fret, two octaves |
The reason the bottom two are three frets rather than two is the B string. The guitar is tuned in fourths except for the gap between the third and second strings, which is a third, and every shape on the neck bends around that one irregularity.
Why this is the thing to learn first
Because it is what a position shift actually is. When you move from one CAGED shape to the next you are moving to another instance of the same root; the octave shape is the move. Learn the lattice and the five shapes stop being five diagrams and start being five places to stand.
There is also a shortcut hidden in it. In every one of the five shapes, in both major and minor, the root is the lowest note the shape plays. So the lowest note under your fingers names the chord, in all sixty shape and key combinations, without counting anything.