This morning, out of the blue, it occurred to me to actually take a structured look at the second, third and fourth natural harmonics (that is, harmonics on frets 7, 5, and 4) on the GAD strings and see what, if anything, it told me. (I've been improvising by ear for a little while now, but hadn't really analyzed the "just what notes are available?" question.) So I wrote it out.
How about that complete D scale? (And by extension, notes in the mixolydian mode of A, lydian mode of G and aeolian mode of B.) And if the tuning is actually DGAD, that D scale can be sounded entirely in the same octave, with the major third (F#, in this case) available on the fourth string as well as on the first.
Brain spinny. Cogitation and experimenting to follow.
Hm. Now I'm wondering about the use of two Xtenders, one on the fourth string and one on the third. A basic tuning of Bb1-F2-D3-G3-A3-D4, with the fourth string dropping to C3 and the third dropping to F3, would seem to give lots of options:
- BbFDGAD. With partial capo 110000, it's D/B with some cool options for Bm. With partial capo 220000, it's CGDGAD. With partial capo 440000, it's DADGAD. The tradeoff would seem to be dealing with the major sixth interval between fifth and fourth strings up the neck, which would be disruptive to scale runs. That is: this tuning would probably dedicate the two bass strings to a bass function, and focus on both harmonic and melodic figures out of the DGAD strings.
- BbFCGAD. This is simply CGDABE a whole step down, by which I am still very intrigued. A full set of four strings in fifths allows fifths-thinking for melody (including the separated fifth on strings 3 and 1), expansive chords in the bass and GAD chords on top. The only things this would seem to give up are the conveniences of that "D-scale-in-harmonics" and the comfortable root-on-top seventh shape that DGAD provides.
- BbFCFAD. CFAD is standard-tuning intervals a whole step down, and brings the conveniences of a Dm triad on the top three strings and the relative major (F) triad on 4-3-2.
- BbFDFAD. This may make a great Dm tuning, with multiple inversions of Dm available on open strings.