Major thirds tuning

The M3 is tuned in fourth fret all the way up so from low to high the open strings are E-Ab-C-E-Ab-C. A crucial property of the M3 is that the symmetric tuning causes runs and chords to form highly structured patterns on the neck. There is no equivalent to the exceptional interval between the 2nd and 3rd string (four semitones as opposed to five semitones between all other pairs of strings) that breaks the symmetry of the conventional tuning.

Advantages

• The M3 is symmetric. A certain pattern, or chord shape, looks the same everywhere on the neck, regardless of the position and the string group.
• On the M3 octaves repeat. Since 3 times 4 is twelve, there is one octave between the same frets three strings apart. You can make chord inversions just by moving one finger three strings up or down! The fourths tuning, which is used by some guitarists to create symmetry, does not have this property.
• On the M3 you play fresh sounds. The M3 is very different to play mechanically from the six-string in conventional tuning so you won't be able to follow the same line of thinking as 99% of the world's guitarists even if you would like to. For this reason it is easier to figure out things that sound new and fresh.

Disadvantages

• You start from scratch. In the beginning, the instrument is totally impossible to play. It probably takes between three and six months before it starts to make sense, and there is a risk that at some point you won't be able to play in conventional tuning, and that might happen before you have got comfortable with the new tuning!
• You throw away part of the tradition of the guitar. A lot of the things you hear guitarists play on records are very hard to execute on the M3, and there are no tutorials that can help you to learn those. Books that include fingerings and tablature are useless so you have to work quite hard, particularly in the beginning, in order to develop your own musical vocabulary.
• You loose four semitones in the high end. Since the highest string is a C instead of an E you might find it difficult to reach the high notes.
• You have to make an effort to find strings. Strings in the right gauges are not readily available, and you will most likely have to buy them on the internet.