Standard tuning is E A D G B E, low to high. Four of those gaps are a perfect fourth and one — G to B — is a major third, and that single irregularity is responsible for most of what makes the guitar neck feel harder to map than a piano. A shape that works on the bottom four strings shifts by a fret when it crosses the G string.
The diagram below computes around that rather than asking you to remember it.
How to read it
The nut is on the left. The column left of the double line is the open strings; the numbers along the bottom are fret numbers. The bottom row is the low E string, the top row is the high E, which is the way tab is written and the opposite of how the guitar looks when you glance down at it.
Orange is the root. Blue is everything else in the scale. When you pick a CAGED position, the notes belonging to that position’s chord are ringed — those are the notes to land on if you want the line to sound resolved rather than merely correct.
Switch to scale degrees if the note names are getting in the way. Reading
1 ♭3 4 5 ♭7 instead of A C D E G is what eventually lets you move a scale to
a new key without relearning it, because the degrees are the same in every key
and the letters are not.
Tap any note to make it the new root. It is the fastest way to see how a pattern moves: find a shape you know, tap a note three frets up, and watch the same pattern arrive somewhere else.
Why the spelling matters
A major scale uses each letter name exactly once. That is not pedantry — it is what makes written music readable, and it is why F♯ major contains E♯ rather than F, and why D♯ major would need an F𝄪 that nobody wants to read.
This tool spells every key properly and tells you when you have picked one that musicians avoid. Choose D♯ major and the readout says it is usually written E♭ major: same notes, same frets, six fewer accidentals on the page.
The neck repeats
Fret 12 sounds the open string an octave up, so the top half of the neck is the bottom half again. Everything you learn below fret 12 you already know above it. On a phone this page draws twelve frets rather than twenty-four for exactly that reason — nothing is missing, only the second copy.
Where to go next
The minor pentatonic is the scale to own first, and its five positions are the ones most players learn as disconnected boxes. CAGED is what connects them: five moveable chord shapes that cover the neck, each one with a scale position wrapped around it.
If you also play something with four strings, the same engine draws bass, ukulele and cavaquinho necks.