Guitar tunings

Guitar tunings are the assignment of pitches to the open strings of guitars, including acoustic guitars, electric guitars, and classical guitars. Tunings are described by the particular pitches that are made by notes in Western music. By convention, the notes are ordered and arranged from the lowest-pitched string (i.e., the deepest bass sounding note) to the highest-pitched string (i.e.the highest sounding note), or the thickest string to thinnest, or the lowest frequency to the highest. This sometimes confuses beginner guitarists, since the highest-pitched string is referred to as the 1st string, and the lowest-pitched is the 6th string.
Standard tuning defines the string pitches as E, A, D, G, B, and E, from the lowest pitch (low E2) to the highest pitch (high E4). Standard tuning is used by most guitarists, and frequently used tunings can be understood as variations on standard tuning. To aid in memorising these notes, mnemonics are used, for example, Eddy Ate Dynamite, Good Bye Eddy.
The term "guitar tunings" may refer to pitch sets other than standard tuning, also called "nonstandard", "alternative", or "alternate". There are hundreds of these tunings, often with small variants of established tunings. Communities of guitarists who share a common musical tradition often use the same or similar tuning styles.
Standard and alternatives.
Standard.
Standard tuning is the tuning most frequently used on a six-string guitar and musicians assume this tuning by default if a specific alternate (or scordatura) is not mentioned. In scientific pitch notation, the guitar's standard tuning consists of the following notes: 
The guitar is a transposing instrument; that is, music for guitars is notated one octave higher than the true pitch. This is to reduce the need for ledger lines in music written for the instrument, and thus simplify the reading of notes when playing the guitar.
Standard tuning provides reasonably simple fingering (fret-hand movement) for playing standard scales and basic chords in all major and minor keys. Separation of the first (high E) and second (B) string, as well as the separation between the third (G), fourth (D), fifth (A), and sixth (low E) strings by a five-semitone interval (a perfect fourth) allows the guitarist to play a chromatic scale with each of the four fingers of the fretting hand controlling one of the first four frets (index finger on fret 1, little finger on fret 4, etc.) only when the hand is in the first position.
The open notes of the second (B) and third (G) strings are separated by four semitones (a major third). This tuning pattern of (low) fourths, one major third, and one fourth was inherited by the guitar from its predecessor instrument, the viol. The irregular major third breaks the fingering patterns of scales and chords, so that guitarists have to memorize multiple chord-shapes for each chord. Scales and chords are simplified by major thirds tuning and all-fourths tuning, which are regular tunings maintaining the same musical interval between consecutive open-string notes. 
Alternative.
Alternative ("alternate") tuning refers to any open-string note arrangement other than standard tuning. These offer different kinds of deep or ringing sounds, chord voicings, and fingerings on the guitar. Alternative tunings are common in folk music where the guitar may be called upon to produce a sustained note or chord known as a drone. This often gives folk music its haunting and lamenting ambience due to the atmosphere and mood that the notes make. Alternative tunings change the fingering of common chords when playing the guitar, and this can ease the playing of certain chords while simultaneously increase the difficulty of playing other chords.
Some tunings are used for particular songs, and may be named after the song's title. There are hundreds of these tunings, although many are slight variations of other alternate tunings. Several alternative tunings are used regularly by communities of guitarists who share a common musical tradition, such as American folk or Celtic folk music. 
The various alternative tunings have been grouped into the following categories:
Joni Mitchell is known for developing a shorthand descriptive method of noting guitar tuning where 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.
String gauges.
String gauge refers to the thickness and diameter of a guitar string, which influences the overall sound and pitch of the guitar depending on the guitar string used. Some alternative tunings are difficult or even impossible to achieve with conventional guitars due to the sets of guitar strings, which have gauges optimized for standard tuning. With conventional sets of guitar strings, some higher tunings increase the string-tension until playing the guitar requires significantly more finger-strength and stamina, or even until a string snaps or the guitar is warped. However, with lower tunings, the sets of guitar strings may be loose and buzz. The tone of the guitar strings is also negatively affected by using unsuitable string gauges on the guitar.
Generally, alternative tunings benefit from re-stringing of the guitar with string gauges purposefully chosen to optimize particular tunings by using lighter strings for higher-pitched notes (to lower the tension of the strings) and heavier strings for lower-pitched notes (to prevent string buzz and vibration).
Dropped tunings.
A dropped tuning is one of the categories of alternative tunings and the process starts with standard tuning and typically lowers the pitch of ("drops") only a single string, almost always the lowest-pitched (E) string on the guitar.
The drop D tuning is common in electric guitar and heavy metal music. The low E string is tuned down one whole step (to D) and the rest of the strings remain in standard tuning. This creates an "open power chord" (three-note fifth) with the low three strings (DAD).
There also exists Double-drop D tuning, in which both E strings are down-tuned a whole step (to D). The rest of the strings keep their original pitch.
Open tunings.
An open tuning allows the guitarist to play a chord by strumming the open strings (no strings fretted).
Open tunings may be "chordal" or "modal". In chordal open tunings, the open chord consists of at least three different pitch classes. In a given key, these are the root note, its 3rd and its 5th, and may include all the strings or a subset. The tuning is named for the base chord when played open, typically a major chord, and all similar chords in the chromatic scale are played by barring all strings across a single fret. Open tunings are common in blues and folk music. These tunings are frequently used in the playing of slide and lap-slide ("Hawaiian") guitars, and Hawaiian slack key music. A musician who is well-known for using open tuning in his music is Ry Cooder, who uses open tunings when playing the slide guitar.
Most modern music uses equal temperament because it facilitates the ability to play the guitar in any key—as compared to just intonation, which favors certain keys, and makes the other keys sound less in tune.
Repetitive open-tunings are used for two classical non-Spanish guitars. For the English guitar, the open chord is C major (C–E–G–C–E–G); for the Russian guitar, which has seven strings, it is G major (D–G–B–D–G–B–D).
When the open strings constitute a minor chord, the open tuning may sometimes be called a "cross-note" tuning.
Major key tunings.
Major open-tunings give a major chord with the open strings.
Open tunings often tune the lowest open note to C, D, or E and they often tune the highest open note to D or E; tuning down the open string from E to D or C avoids the risk of breaking strings, which is associated with tuning up strings.
Open D.
The open D tuning (D–A–D–F–A–D), also called "Vestopol" tuning, is a common open tuning used by European and American/Western guitarists working with alternative tunings. The Allman Brothers instrumental "Little Martha" used an open-D tuning raised one half step, giving an open E♭ tuning with the same intervalic relationships as open D.
Open C.
The English guitar used a repetitive open-C tuning (with distinct open notes C–E–G–C–E–G) that approximated a major-thirds tuning. This tuning is evident in William Ackerman's song "Townsend Shuffle", as well as by John Fahey for his tribute to Mississippi John Hurt.
The C–C–G–C–E–G tuning uses some of the harmonic sequence (overtones) of the note C. This overtone-series tuning was modified by Mick Ralphs, who used a high C note rather than the high G note for "Can't Get Enough" on "Bad Company". Ralphs said, "It needs the open C to have that ring," and "it never really sounds right in standard tuning".
Open G.
Mick Ralphs' open-C tuning was originally an open-G tuning, which listed the initial six overtones of the G note, namely G–G–D–G–B–D; Ralphs used this open-G tuning for "Hey Hey" and while writing the demo of "Can't Get Enough".
Open-G tuning usually refers to D–G–D–G–B–D.
The open G tuning variant G–G–D–G–B–D was used by Joni Mitchell for "Electricity", "For the Roses" and "Hunter (The Good Samaritan)". Truncating this tuning to G–D–G–B–D for his five-string guitar, Keith Richards uses this overtones-tuning on The Rolling Stones's "Honky Tonk Women", "Brown Sugar" and "Start Me Up".
The seven-string Russian guitar uses the open-G tuning D–G–B–D–G–B–D, which contains mostly major and minor thirds.
Creating any kind of open tuning.
Any kind of chordal tuning can be achieved, simply by using the notes in the chord and tuning the strings to those notes. For example, Asus4 has the notes A, D, E. By tuning the strings to only those notes, it creates a chordal Asus4 tuning.
Bass players may omit the last two strings.
Minor or "cross-note" tunings.
Cross-note tunings include a minor third, so giving a minor chord with open strings. Fretting the minor-third string at the first fret produces a major-third, so allowing a one-finger fretting of a major chord. By contrast, it is more difficult to fret a minor chord using an open major-chord tuning.
Bukka White and Skip James are well-known for using cross-note E-minor in their music.
Other open chordal tunings.
Some guitarists choose open tunings that use more complex chords, which gives them more available intervals on the open strings. C6, E6, E7, E6/9 and other such tunings are common among lap-steel players such as Hawaiian slack-key guitarists and country guitarists, and are also sometimes applied to the regular guitar by bottleneck (a slide repurposed from a glass bottle) players striving to emulate these styles. A common C6 tuning, for example, is C–E–G–A–C–E, which provides open major and minor thirds, open major and minor sixths, fifths, and octaves. By contrast, most open major or open minor tunings provide only octaves, fifths, and either a major third/sixth or a minor third/sixth—but not both. Don Helms of Hank Williams band favored C6 tuning; slack-key artist Henry Kaleialoha Allen uses a modified C6/7 (C6 tuning with a B on the bottom); Harmon Davis favored E7 tuning; David Gilmour has used an open G6 tuning.
Modal tunings.
Modal tunings are open tunings in which the open strings of the guitar do not produce a tertian (i.e., major or minor, or variants thereof) chord. The strings may be tuned to exclusively present a single interval (all fourths; all fifths; etc.) or they may be tuned to a non-tertian chord (unresolved suspensions such as E–A–B–E–A–E, for example). Modal open tunings may use only one or two pitch classes across all strings (as, for example, some metal guitarists who tune each string to either E or B, forming "power chords" of ambiguous major/minor tonality).
Popular modal tunings include D Modal (D-G-D-G-B-E) and C Modal (C-G-D-G-B-D).
Lowered (standard).
Derived from standard EADGBE, all the strings are tuned lower by the same interval, thus providing the same chord positions transposed to a lower key. Lower tunings are popular among rock and heavy metal bands. The reason for tuning down below the standard pitch is usually either to accommodate a singer's vocal range or to get a deeper/heavier sound or pitch. Common examples include:
E♭ tuning.
Rock guitarists (such as Jimi Hendrix on the songs Voodoo Child (Slight Return) and Little Wing) occasionally tune all their strings down by one semitone to obtain "E♭ tuning". This makes the strings easier to bend when playing and with standard fingering results in a lower key.
D tuning.
D Tuning, also called One Step Lower, Whole Step Down, Full Step or D Standard, is another alternative. Each string is lowered by a whole tone (two semitones) resulting in D-G-C-F-A-D. It is used mostly by heavy metal bands to achieve a heavier, deeper sound, and by blues guitarists, who use it to accommodate string bending and by 12-string guitar players to reduce the mechanical load on their instrument. Among musicians, Elliott Smith was known to use D tuning as his main tuning for his music. It was also used for several songs on The Velvet Underground's album The Velvet Underground & Nico.
Regular tunings.
In standard tuning, there is an interval of a major third between the second and third strings, and all the other intervals are fourths. The irregularity has a price. Chords cannot be shifted around the fretboard in the standard tuning E–A–D–G–B–E, which requires four chord-shapes for the major chords. There are separate chord-forms for chords having their root note on the third, fourth, fifth, and sixth strings. These are called inversions.
In contrast, regular tunings have equal intervals between the strings, and so they have symmetrical scales all along the fretboard. This makes it simpler to translate chords. For the regular tunings, chords may be moved diagonally around the fretboard. The diagonal movement of chords is especially simple for the regular tunings that are repetitive, in which case chords can be moved vertically: Chords can be moved three strings up (or down) in major-thirds tuning, and chords can be moved two strings up (or down) in augmented-fourths tuning. Regular tunings thus appeal to new guitarists and also to jazz-guitarists, whose improvisation is simplified by regular intervals.
On the other hand, five- and six-string open chords ("cowboy chords") are more difficult to play in a regular tuning than in standard tuning. Instructional literature uses standard tuning. Traditionally a course begins with the hand in first position, that is, with the left-hand covering frets 1–4. Beginning players first learn open chords belonging to the major keys C, G, and D. Guitarists who play mainly open chords in these three major-keys and their relative minor-keys (Am, Em, Bm) may prefer standard tuning over many regular tunings, On the other hand, minor-thirds tuning features many barre chords with repeated notes, properties that appeal to acoustic-guitarists and beginners.
Major thirds and perfect fourths.
Standard tuning mixes a major third (M3) with its perfect fourths. Regular tunings that are based on either major thirds or perfect fourths are used, for example, in jazz.
All fourths tuning E2–A2–D3–G3–C4–F4 keeps the lowest four strings of standard tuning, changing the major third to a perfect fourth. Jazz musician Stanley Jordan stated that all-fourths tuning "simplifies the fingerboard, making it logical".
Major-thirds tuning (M3 tuning) is a regular tuning in which the musical intervals between successive strings are each major thirds, for example E2–G2–C3–E3–G3–C4. Unlike all-fourths and all-fifths tuning, M3 tuning repeats its octave after three strings, which simplifies the learning of chords and improvisation. This repetition provides the guitarist with many possibilities for fingering chords. With six strings, major-thirds tuning has a smaller range than standard tuning; with seven strings, the major-thirds tuning covers the range of standard tuning on six strings.
Major-thirds tunings require less hand-stretching than other tunings, because each M3 tuning packs the octave's twelve notes into four consecutive frets. The major-third intervals let the guitarist play major chords and minor chords with two–three consecutive fingers on two consecutive frets.
Chord inversion is especially simple in major-thirds tuning. The guitarist can invert chords by raising one or two notes on three strings—playing the raised notes with the same finger as the original notes. In contrast, inverting triads in standard and all-fourths tuning requires three fingers on a span of four frets. In standard tuning, the shape of an inversion depends on involvement of the major-third between the 2nd and 3rd strings.
All fifths and "new standard tuning".
All-fifths tuning is a tuning in intervals of perfect fifths like that of a mandolin or a violin; other names include "perfect fifths" and "fifths". It has a wide range. Its implementation has been impossible with nylon strings and has been difficult with conventional steel strings. The high B makes the first string very taut, and consequently a conventionally gauged string easily breaks.
Jazz guitarist Carl Kress used a variation of all-fifths tuning—with the bottom four strings in fifths, and the top two strings in thirds, resulting in B1–F2–C3–G3–B3–D4. This facilitated tenor banjo chord shapes on the bottom four strings and plectrum banjo chord shapes on the top four strings. Contemporary New York jazz-guitarist Marty Grosz uses this tuning.
All-fifths tuning has been approximated by the so-called "New Standard Tuning" (NST) of King Crimson's Robert Fripp, which NST replaces all-fifths' high B4 with a high G4. To build chords, Fripp uses "perfect intervals in fourths, fifths and octaves", so avoiding minor thirds and especially major thirds, which are slightly sharp in equal temperament tuning (in comparison to thirds in just intonation). It is a challenge to adapt conventional guitar-chords to new standard tuning, which is based on all-fifths tuning. Some closely voiced jazz chords become impractical in NST and all-fifths tuning.
Instrumental tunings.
These are tunings in which some or all strings are retuned to emulate the standard tuning of some other instrument, such as a lute, banjo, cittern, mandolin, etc. Many of these tunings overlap other categories, especially open and modal tunings.
Miscellaneous or "special" tunings.
This category includes everything that does not fit into any of the other categories, for example (but not limited to): tunings designated only for a particular piece; non-western intervals and modes; micro- or macro-tones(half sharps/flats, etc.); and "hybrid tunings" combining features of major alternate tuning categories – most commonly an open tuning with the lowest string dropped.