Dulcimer tuning problem
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Great information! I didn't know until now about just intonation or natural temperament. I read through the linked discussion thread about JI and that describes it perfectly. And yes, the length of the first fret is less than the length of the second so I believe that confirms it. I'm pretty sure when I was first learning to play years ago, I was playing melody/drone style and not chords so I wouldn't have noticed it then. I'll also try tuning to DAA to see if that makes a difference.
Thanks, everyone, for all your help! I learned a lot from this discussion.