DddA is switching the strings from D3-A3-d4-d4 [standard DAdd] so the sizes would stay about the same, D3-.022W, A3-.012, d4- .010, adjusted to VSL.

Note that if one assumes an average dulcimer VSL (distance from nut to bridge) of around 27", one would need an impossibly thin string to tune way up to the 'a' note above high d (designated as D4, right above middle C). I believe any string would break before reaching that high of a note at that scale length, no matter what the gauge. So, on a normal sized dulcimer, any string tuned to the note of A would have to be tuned to an A note lower than middle C (C4).

Using a low wound D bass string, and a thin (.010?) unwound middle string in high d, and another thin or medium gauge string (.012/0.13?) for the melody string tuned to A (three notes below the high d)... would enable you to play in the key of D in ionian mode in noter style.  The tuning would be best labeled as DdA (I might have mistakenly typed it as Dda).  Fretting the melody string on the 3rd fret would then make it sound like a unison tuning of Ddd.  It's like DAA but without the '5th' on the middle string. It would have a unison tuning sound like Ddd, but would also give you the three extra lower notes to play on the melody string.  (It's late and I hope I'm not confusing anything in my explanation!)

@tonyg Indeed, I did.  Thanks for pointing out my error-- I'll correct the posting.  And hope I use the correct "A".  I'm a dunce when writing/talking about music.   

Robin......did you mean Ddda?

I play DddA (using doubled high drones) and like it lots.  I've been stringing instruments for awhile with a wound bass, doubled high drones, and single melody string.  

Never heard of that tuning, but if the middle string is an octave above the bass string--which is indicated by the lower case "d"--it probably has to be plain steel.

Dda........you don't hear much about this tuning.  Strumelia mentioned it in an old blog, and I was wondering if the middle string would be an unwound high d or a wound lower D like the bottom string?  Also, any thoughts or experiences with this tuning for drone style would be appreciated.