A neck diagram shows a scale or chord the way your hand meets it: as dots on strings and frets. Pick a root and a scale, and every note of that scale lights up across the whole neck, in any key. Switch to chord notes to see where every C, E, G and B of Cmaj7 sits, which is how players find arpeggios and new chord shapes; or show all twelve notes counted from a root.
Label the dots the way you think: note names spelled correctly for the key (B♭ in F major, not A♯), scale degrees (1, ♭3, 5), intervals (R, m3, P5) or sargam (S R G M P D N), or switch the labels off and learn the shapes. The root, third, fifth and seventh each get their own colour, so the chord inside a scale stands out.
Tap a note in the strip above the neck to hide it, for example to see only the 1, 3 and 5 of a scale, or the pentatonic notes inside the major scale. Tap any spot on the neck to hear it and see how it relates to the root, or press Play to hear every note in view, low to high and back.
Zoom with a pinch, Ctrl or ⌘ and the scroll wheel, or the buttons, and choose a fret window from the whole neck down to a single position. Any tuning works, including drop and open tunings, 7-string, bass and ukulele. Save a picture, print a sheet, or print a blank neck for your own diagrams.