This is a single key major scale (do-re-mi) autoharp. Since diatonics use 7 notes of the scale rather than 12, there are strings left over which can be doubled. One must decide which notes to double, how to structure the bass octave, what chords can be made (and whether partial 7ths are desired), etc. There is plenty of instruction out on the web as to how to do this.

The main difference with this system is that there must be two strings for the second step of the scale, and they are tuned to slightly different pitches. By experimenting some, I concluded that a variance of 8 cents is tolerable (because if you employ an open note technique, occasionally the two strings are going to be sounded together), but that difference is arbitrary and it could be adjusted one way or the other.

The reason for this is that I want the higher second step string to fit better as a fifth in the V chords, and the lower one to fit better as a fifth in the II chords. That means your chord bars must be cut to accommodate this. In other words, given a C major scale where the d note is the second step, you open the higher d for the G chord, and the lower one for the Dm chord

You do not really have to understand why, you can just apply the temperament, transposing as desired, listed in the Tuning section here. If you want to delve into the physics of it, I have uploaded an essay, the first draft of which was written in 1988 for a workshop. I think pretty much everyone's eyes glazed over about two minutes into that class, so I learned my lesson and just kept subsequent modifications to myself. Well, until now, I guess.

[Be advised that in the essay, which was written atdifferent times, the exact number of cents in various schemes is not consistent. Don't get hung up on that. Just realize that the relative values are what I am trying to illustrate.]

If you are curious, my diatonics have the following chord arrangement:

V7                    IIm                    VIm                    IIIm 

 II7 partial                    IV                     I                           V

I am used to two rows with the minors on the treble side, and I play open note-style quite a bit. But that's purely personal preference.