Big thanks to Ant Law for letting me use some text and diagrams from his book on P4 Tuning and making the video about it too. Do go check out his record, Life I Know.

Perfect 4ths tuning (sometimes called P4) just means tuning every string to the 5th fret of the string below which gives us the interval of a Perfect 4th between every string. It's very close to standard tuning, which is nearly the same, except for the major 3rd interval between the G and B-strings.

This results in two main kinds of 4ths tuners. Some tune the top 2 strings up a semitone so they have (low to high) E, A, D, G, C, F. Ant chooses to go down in fourths from the high E-string, giving me (low to high) Eb, Ab, Eb, Gb, B, E.

P4 works great for copying shapes across the fretboard and can be really intuitive, as we don't have to make any adjustment to compensate for the major 3rd interval that disrupts the symmetry. It's really easy to copy shapes and ideas across the neck, and can be much quicker learning where all the notes are, and learning chords etc.

Ant Law's Book on 3rds Tuning: 3rd Millenium Guitar (on Amazon)

Case Study 1

Take this scale in normal tuning. Excluding variations in timbre, all 3 sound exactly the same.

They look quite different. But what if they all looked identical? They sound the same, shouldn’t they look the same too? In P4 Tuning, they do!

We see that the symmetry is a huge advantage. It is much easier to visualise the shape. We can move it around the guitar without much effort. This simplicity is why Violins, Violas, Cellos, Double Basses, Bass guitar, the Piano etc are all organised symmetrically.

Case Study 2

Any guitar player tuned in fourths has to learn only 33.33% of what someone tuned normally does. Look at these three ways of playing a D major chord. To play the same chord you need different fingerings for each area of the neck, different stretches and a lot more to remember.

Now look at these 3 ways in fourths tuning.

Remember that many instruments are set up like this, or at least in a similar symmetric fashion, particularly the string family and keyboard instruments. The guitar is the odd one out.