7. Now it is time to learn the relation between frets or rather between groups: knowledge of groups comes very handy. We should start with the most important music intervals: thirds, major and minor. Thirds are represented either SS or LL, see the example on the left.
Within the same group, an interval between notes of strings 2 and 3 is a third. For other pairs of strings: 1 and 2, 3 and 4, 4 and 5, 5 and 6, the interval between notes is a third for neighboring groups. Practice thirds on the C major PAD going up and down the fretboard along the strings or across. When notes are memorized, remove the visual tool and play thirds envisioning the note symbols in your mind. When you are ready, check yourself with the tests. Finally, play the thirds in any order all around the fingerboard.
Here is another testing exercise with all 35 thirds of guitar across the strings in C major key for frets 0-12 played in unison on adjacent groups of frets. "Not unison" thirds are also mentioned for the reference. It starts from fret 0 to 12. Play the natural music of guitar in thirds ("4" marks major 3-rd intervals, other are minor 3-rds):
