Hi Helen...I've been trying ot post to this thread all day and (grumble, grumble) I keep having to reset my cookie!!
Anywhoooo, my favorite diatonic D autoharp for many fiddle tunes has thirty seven strings and is tuned like this:
gadef#gabc#ddeef#f#ggaabbc#c#ddeef#f#ggaabc#dd
It has fifteen bars and the chords are set up this way:
G9 D4 D6/9 A4 A6/9 Emin F#7 Bmin C#dim F#dim G A7 D E7 A

The F#7 and E7 are partial chords -- no third
9ths are added - no 7ths
4 means suspended 4th
The Dadded6/9 chord = D major pentatonic scale and you can then push two chords for various drones.
I don't take credit for it though. It's something Marty Schuman and Mark Fackeldy worked out for me...Bill Sables asked about right hand work...I'll send you a note Bill when I find a moment!...harpy