There is no "rule of thumb" for any specific tuning which would correspond best to any specific song.
The
most we can say is that a tuning may be useful to play a specific song, a specific arrangement of a song, or in
a
specific key.
Sometimes it is better to stick to one particular non-standard tuning, and experiment with it. When
experimenting,
try to play different songs that you would have been unable to play in standard tuning. Look at the following
criteria for guidance:
Look at the key. If you find out what key the song is in, you can use that as a guide for
what
notes you want to have open strings tuned to. It is often the purpose of alternate tunings to make sure that
the
open strings are tuned to notes that are frequently used in the song.
Generally, the root and fifth of the key should be highly accessible. If the key of the
song is
"D Major", then the notes "D" and "A" will usually be easy to play using open
strings. In most cases, the root and fifth note of a key can anchor the song to a particular key, so they
can be
used quite freely throughout the song. The standard tuning of a guitar is EADGBe, so if a song is in D, the
lower E is often tuned down one whole-step to D, resulting in DADGBe. This is probably the most common
alternate
tuning, and is naturally suited to playing in the key of D.
Look at repeated bass lines or melodic motifs. If there is some repeated theme, bass-line,
or
other melodic fragment, the strings of the guitar can be retuned to make playing this motif more convenient,
thus freeing you to do something more exciting and interesting with the remaining strings. In the example of
DADGbe above, let's say the motif is a low "D" note followed by the "A" one fifth
above.
You can play this motif simply by playing the low 6th string and the low 5th string. That doesn't
require
the use of your left hand at all, and now your left hand is free to explore other ideas.
You can approach from three base tunings; any of which can be further altered by one or more strings. Each tuning
has
its own attributes:
Standard: if the song has lots of modulation, is really chromatic or has a really active
melody
then this key is easier to negotiate. A lowered e-d or a-g can help round out the low end if you need to.
DADGAD: Really lends itself to chords that have high extensions. You can get interesting
voicings and a nice resonant sound without the strings going too 'slappy'. Bonus: you can still
negotiate lots of key changes and chromaticism - you don't have to rely on open strings as much as you
might
think. It has many different intervals of 3rds and 5ths on the same fret, so it is easier to tap with the
left
hand, like Mike Dawes (a fingerstyle guitarist).
CGCGCE: When you just want to focus on rhythmic bass vs melody, this is a good place to
start.
It's harder to get big, lush chord voicings but the chord changes can easily be implied.
If this is not helping, then you should just start by learning some songs in different tunings, take Kashmir (Led
Zepplin) for example. It's definitely a good idea to experiment with different alternate tunings, but until
you
can play a few songs in different tunings, the use of alternate tunings may not seem clear to you.
