Open Tuning

Regular tunings
For regular guitar-tunings, the distance between consecutive open-strings is a constant musical-interval, measured by semitones on the chromatic circle. The chromatic circle lists the twelve notes of the octave.
Basic information
Aliases	Uniform tunings
Advanced information
Advantages	Simplifies learning by beginners and improvisation by advanced guitarists
Disadvantages	Replicating the open chords (“cowboy chords”) of standard tuning is difficult; intermediate guitarists must relearn the fretboard and chords.
Regular tunings (semitones)
Trivial (0)
Minor thirds (3)
Major thirds (4)
All fourths (5)
Augmented fourths (6)
New standard (7, 3)
All fifths (7)
Minor sixths (8)
Guitar tunings

Guitar tunings assign pitches to the open strings of guitars, including acoustic guitars, electric guitars and classical guitars, among others. Tunings can be described by the particular pitches that are denoted by notes in Western music. By convention, the notes are ordered from lowest-pitched string (i.e., the deepest bass note) to the highest-pitched (thickest string to the thinnest).

The phrase “guitar tuning” also refers to the adjusting of the string-pitches to their desired tuning to a reference pitch–often a note from a piano or Hammond organ and/or tuning the guitar strings so that the strings are in tune relative to each other. Tuning is described in how-to manuals for guitarists.

Standard tuning defines the string pitches as E, A, D, G, B, and E, from lowest (low E) to highest (high E). Standard tuning is used by most guitarists, and frequently used tunings can be understood as variations on standard tuning. “Nonstandard” tunings are also called “alternative” or “alternate”. Some tunings are used for particular songs by professional musicians, and may be called after the song's title. There are hundreds of such tunings, which are often minor variants of established tunings. Communities of guitarists who share a musical tradition often use the same or similar tunings.

The hundreds of alternative tunings have been classified into a smaller number of categories: “open”, both major and minor (“crossnote”), and “modal”; “dropped” (in which the pitch of one or more strings is lowered); “instrumental” (based on other stringed instruments); and “regular”. Modal, dropped, and many other tunings are mentioned in the supplementary list of guitar tunings.

Joni Mitchell developed a shorthand descriptive method of noting guitar tuning wherein the first letter documents the note of the lowest string and is followed by the relative fret (half step) offsets required to obtain the pitch of the next [higher] string This scheme highlights pitch relationships and simplifies the process of comparing different tuning schemes.