It's worth reading Robert Force's story about the book:
https://robertforce.com/SongsAndInstruction/InSearchOfTheWildDulcimer.html
And you can read the book itself online here:
https://robertforce.com/SongsAndInstruction/InSearchOfTheWildDulcimer-PDFs.html
NOTES: in the "Tuning chart" page, at bottom he states the string length as being 24"... quite a short vsl.
He also states the strings as being listed 'from low to high', therefore he writes his mixolydian tuning in the chart from top to bottom: DADD meaning the top D is the low bass string and the two bottom d's are the pair of melody strings. He refers to the melody string s , plural. He lists the gauges as 22(low bass), 12, and 10 (0.22, 0.12, and 0.10)
He's giving DGdd as the tuning for Ionian mode. That's a 'reverse Ionian' tuning for the key of G.
Why that somewhat unusual tuning for playing in ionian mode? I figure this came about because when he tuned to DAAA on a 24" scale with the melody strings being 0.10, they were just too floppy for him to think playable. The third fret on the melody strings in DGdd is a G note, so that's the tonic/key he indicated. But if you tried to tune typically (1-5-5) for ionian in the key of G, you'd tune Gddd. The Bass string would break long before hitting that higher G, and it'd likely be too loose and jangly if tuned to the G below the usual D. To avoid breaking the bass or middle string is the reason some folks use a 'reverse tuning'- where the tonic low note is instead on the middle string and the heavy string is tuned instead to a fifth below the tonic. The bass and middle string notes get swapped. It saves strings from breaking or from being too low to play.
For the key of G that reverse tuning would then be DGdd as in the chart, with the middle string taking over the job of the tonic low note, the melody strings playing high g tonic on the 3rd fret, and the bass string tuned to a fifth from the tonic (the 5th note in the key of G being a D) but in the lower octave. The bass and middle strings have exchanged duties, and you don't break any strings.
If the scale length of the person mapping out the tuning chart had been a more typical 26-28" length, then the normal key of D ionian tuning of DAAA would have worked fine. OR, it also would have also worked fine in DAAA on a 24" scale if he had put heavier strings on.
It's little wonder folks tuned up from the key of C to the key of D if they were making or playing 24" scale dulcimers. I also notice he has the chart use the key of E for aeolian mode. I think a lot of this was to accommodate that short 24" scale whoever wrote the chart was working with, assuming they didn't want to try changing to heavier strings for whatever reasons.
Robert's intro clearly states that he (and others he was working with) didn't really know much of anything about dulcimers or how they were traditionally played when they started having fun with them. They experimented, improvised, adapted, and learned as they went along.
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