A twelve-string player gave me a good tuning tip for my hammered dulcimer, which he called the "shimmer effect." One string in each pair is tuned correctly, and the other about 5 cents flat. The result is a harmonic mix that creates notes that are not actually played, giving a much fuller and richer sound. A piano-tuner uncle of a lovely local barmaid expanded the technique for me by emphasizing that the first or dominant string in a pair should be tuned correctly, and the other a bit off. I'm familiar with undertones and overtones in choral singing (especially in the "Barbershop" tradition), but I'm not sure whether that's the correct terminology when discussing stringed instruments.