Well, I tried a lot of chord layouts, but logic brought me inexorably to the good ol' Circle O' Fifths. But does the Way Music Works drag you to the Circle, or does the Circle make music work?
There are little pockets of logic in the Oscar Schmidt autoharp layout, but they evaporate pretty quickly. I can only presume that Oscar thought that nobody cared what key they were actually in (i.e., no one ever played with anyone else on a different instrument), and so being able to shift over and play in, say, Ab, seemed like a great idea. This is even more prevalent on the 21 chord model. And clearly, 7th chords were a lot more fashionable in the Olden Days than they are with me, now.
But, if you're gonna play with other people (or, maybe more importantly (to me), use other people's sheet music), you only need to aim at reasonable guitar keys and chords. My layout is gonna let me play in C, G, D, A, and E, with all the normally required chords available. I can't play in any flat or sharp keys, nor in F or B, but I'm pretty sure I can live with that <grin>. Even if my kids need to transpose a song to put it in their singing range, E is close enough to F, and C to B.
That said, it's true that since I'm not gonna use B as a I or IV, it can be a 7th chord. I can at least give it a high-octave 7, and maybe the middle one, too. Seems like the chords towards the left and right edges of the layout are "not gonna be I" chords, and can be 7ths -- like the Dm, too, perhaps. Still, I'm always a lot happier to leave a 7th out than to have one in that doesn't belong. Maybe they all can receive the high-octave-7th-only treatment, just in case. At the least, maybe I should re-scan a stack of songs to see which appears more often, B or B7, Dm or Dm7, C#m or C#m7, etc.
