All fourths tuning for guitar is an alternative tuning for guitar in which the six open strings of the guitar (lowest to highest) are tuned to the following notes: E, A, D, G, C, F.
The most common guitar tuning is EADGBE (lowest to highest). The all fourths tuning can be obtained from the standard tuning by: 1) raising the second guitar string (highest to lowest) one semitone from B to C; and 2) raising the first guitar string one semitone from E to F.
The all fourths tuning is called so as each string is five semitones higher than the lower string and an interval of five semitones is called a perfect fourth. A is the fourth of E, D is the fourth of A, G is the fourth of D, and so on. Thus, unlike the standard tuning where the second string (highest to lowest) is checked against the fourth fret of the third string, each string in the all fourths tuning is checked against the fifth fret of the lower string. This tuning hence produces symmetrical scales up and down the guitar neck and is usually used by double-hand tapping guitarists.