The standard tuning used on guitars, from 6th string to 1st, is E, A, D, G, B, E. This tuning evolved over a long period of time and has been found to give a good range, convenient placing of intervals, and a manageable choice of basic chord fingerings. But there are many other tunings, which are said to be non-standard or alternative. Joni Michell reckoned to have used 51 different tunings in her music over the years.

 

Open tunings

 

Common open and crossnote tunings
open A	E	A	C#	E	A	E
open C	C	G	C	G	C	E
open D	D	A	D	F#	A	D
open G	D	G	D	G	B	D
crossnote D	D	A	D	F	A	D
crossnote E	E	B	E	G	B	E

 

In an open tuning, the six strings are tuned to make a simple major chord. For example, in open D tuning the strings are tuned to D, A, D, F#, A, D. Open tuning makes it easy to play unusual chordal combinations and more open sounding chords by utilizing "drone" and "sustained" strings. Barre chords can be played with only one finger, and open tuning is ideally suited to playing bottleneck slide and harmonics, because all six strings can be sounded.

 

Crossnote tunings

Crossnote tunings are open tunings in which the open strings sound a minor chord instead of a major one. They differ from the ordinary open tunings in that one string is lowered by a half-tone. This gives the minor third interval that is characteristic of a minor chord. For example in crossnote D tuning the 3rd string is tuned down to F instead of F#.

 

Dropped tunings

The simplest and probably the most commonly used alternative tunings, dropped tunings involve lowering the pitch of just one or two strings. The most useful is dropped D, which is the same as standard tuning but with the 6th string dropped down a tone from E to D. Dropped D is ideally suited to playing in the key of D as the open 6th string can be used to play the bass. It's also useful when playing in the key of A because the open 5th string can be used as the bass for the tonic chord of A, while the open 6th string can be used as the bass for subdominant chord of D.

 

A variation on dropped D is to also drop the 2nd string from B to A, so that it plays an octave higher than the 5th string.

 

Modal tunings

Another popular alternative tuning is D, A, D, G, A, D, which is based on open D tuning but, thanks to the 3rd string being tuned to G rather than F#, sounds a D suspended 4 chord with all strings open. This lends an ambiguity to the tuning since the third has been taken out of the open D chord, leaving it neither major nor minor. Tunings such as DADGAD are often referred to as modal tunings. DADGAD is also called Celtic tuning because it is used in a lot of Celtic music, as well as rock. Jimmy Page, for example, of Led Zeppelin and The Yardbirds, used DADGAD extensively.