Other tunings
Introduction
Occasionally, composers for the guitar seem to choose odd tunings; one has to trust that it's not to deliberately confuse those around them. Keith Richards mentioned that alternative tunings help him write songs differently. As another example, John Fahey, who experimented like a mad alchemist with all sorts of tunings, even tried matching all 6-strings to the same pitches, or just 5ths.
Additionally, sometimes songs transcribed here remain as unsolved mysteries, where it seems impossible to physically reach certain notes in Standard, Vestapol, or Spanish tunings.. that's where these custom tunings can work as 'patches', where the string pitches are altered just to accommodate odd sections in songs -- at least until new clues for fingerings help them get re-categorized as more plausible && common tunings.
Finally (though listed first), for classical arrangements, oftentimes drop tunings are used, such as DADGBD, where it's standard tuning, except the bottom-most E-string is reduced by 1-whole-step, which conveniently then matches the 4th-string. Also, classical arrangements seem to sometimes use DGDGBD tuning; this is almost like a "hybrid" of Spanish-tuning (for the bass strings) and Standard-tuning (for the treble strings). In the Spanish tuning way, the tuning gravitates the tonal center to the "G" tonic/root, on the 5th-string, with the D-strings (on the 6th and 4th string) supportive as intervals of 5ths, relative to the G-note.
i) DADGBE
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