Let us again have a look at the fretboard on the M3, with the markings that are symbolic of the numbers 0, 1, 2, and 3. The markings indicate the index of each fret which is given by how close it is to a number that divides by four (technically speaking it is the fret number modulo 4). So frets 0 (open strings), 4, 8, 12... have index zero. Frets 1, 5, 9, 13... have index 1, frets 2, 6, 10, 14... have index 2, and frets 3, 7, 11, 15... have index 3. Now, since the notes mapped out on the fretboard repeat for every four frets along a string and for every three strings along a fret, a particular note always corresponds to a unique index.
Take the note A as an example. Its index is 1. If you are looking for an A on the M3 it will be available in frets 1, 5, 9, 13... and nowhere else. You can go in the opposite direction as well, mapping frets to notes. If you are in fret 6 which notes are available to you? D, Gb, and Bb since fret 6 has index 2.
|Index||Note||Note in clock notation|
|0||Ab, C, E||12, 4, 8|
|1||A, Db, F||1, 5, 9|
|2||Bb, D, Gb||2, 6, 10|
|3||B, Eb, G||3, 7, 11|