After having spent 20 years playing guitar the normal way, (2 of those years I tuned my guitars in major thirds all over), I decided to tune my guitars E A D G C F.
Every scale, arpeggio and melody you care to learn keep the same fingering regardless of which strings you play them on. (The standard tuning needs 3 different fingerings for doing the same job.)
When I improvise I want to play any idea that pops up in my head without thinking about the fingering.
Chords without open strings are movable across the strings, not only along the strings.
All the fingering you already use and know on the E A D G can be used on all strings.
The learning process is pretty much an unlearning process: Simply forget about all the fingering you have done on the G B E strings.
1: The standard barred chords are difficult to grab (but there are lots of new and interesting ones).
2: The basic open string cords do not work, because you lack the B and E.
3: Classic pieces are difficult to play for the same reason.
In order to remove disadvantage number 2 and 3 I did a minor modification to my guitar:
I gave the 2 thinnest strings 1 fret more than the 4 thickest strings.
To do this I simply made 4 deep cuts through the nut and through some centimeters of the fret board next to the nut for the E A D G strings. Now the E A D G strings rest permanently on to the 1. fret.
The thin B and E strings start at the nut as before.
(A guitar workshop can do this for you).
You may also try this out by fixing a capo over the E A D G behind the 1. fret, or putting a match or a piece of wood under the E A D G behind the 1. fret.
If you do the modification, you will possibly prefer to use slightly thicker strings for E A D G, as they have become a bit shorter.
Tune the guitar as indicated on the picture and try this:
1: Listen to all the open strings: They sound the same as standard tuning. E A D G B E
2: Listen to all the strings as you make a barre behind the 1. fret: E A D G C F = P4 tuning.
3: Listen to the basic E major chord (1 fret up of course). Works!
4: Move exactly this chord 1 STRING down: A major!
5: Move exactly this chord another STRING down: D major!
With the P4 tuning and this small modification the basic E. A and D major chords have exactly the same shape!
To simply try out the P4 tuning without this genius modification you may also prefer to tune the 4 thick strings down to Eb Ab Db Gb = Eb Ab Db Gb B E, (instead of up to E A D G C F), as the thin E may break if you tune it to F and even do some string bending (which also is a bit harder to do after you have tuned it up).
I have found a very easy system for learning the western diatonic scales on a P4 guitar: Major scale, Minor scale, Greek Dorian mode, Medieval and Modern Dorian mode and so on, meaning all the scales you can play on the white keys on a piano.
(Slide the pattern up or down the neck to transpose to another key.)
One pattern covers it all:
Imagine a guitar with lots of strings like this one:
Look at the beautiful repeating pattern!
Between two frets there are always 7 dots above each other, then 5 strings with no dot, then 7 dots, and so on. The 2 lower dots always overlap the 2 upper dots at the next fret.
On the lower string I have marked that you play an interval of 1 fret, then another of 2 frets, starting from the left dot.
In this explanation i call this a 12.
21 means playing a major second, then a minor second, from left to right on the fretboard.
22 means playing 2 major seconds after each other from left to right on the fretboard.
The picture above shows a scale consisting of:
12 - 12 - 21 - 21 - 22 - 22 - 22 - (12 - 12 - 21 - 21 - 22 - 22 - 22 ) - and so on, starting bottom left.
Learn this easy repeating sequence of numbers by heart: 12 - 12 - 21 - 21 - 22 - 22 - 22 -
and play it on the guitar. Try to start with a 12, a 21 or a 22, and you get different scales.
A major scale may start here: 22 - 22 - 12 - 12 - 21 - 21 play it upwards and downwards.
Major scales, starting on the dots with a ring around them.
Start somewhere else, and you get another scale. Easy?
With this matrix painted on the fretboard I know immediately where to play any C or G or Ab, Painting the Fretboard
Have a look at the photo of my main guitar.
(The letters are painted on the photo, NOT on the guitar)
This is a part of the same pattern as you have seen on the diagrams with 16 strings.
The white parts are the same as the white keys on a piano.
The brown parts are the same as the black keys on a piano.
(once i have memorized the matrix by heart).