This sweet little fiddle duet (at PopTech) is the best example I've ever heard to illustrate the sound of "just intonation".
With "just intonation", every note relates to every other note in an exact ratio: 1:2, 2:3. 4:5, etc. The brain does the math and identifies such notes as "musical". Fretless stringed instruments and wind instruments necessarily play "just" music. So also the untrained human voice (a child's nursery song). It is a characteristic of traditional music.
Modern (last few centuries) music has invented fretted instruments and keyboard instruments with "tempered intonation". The advantages are: one can play in any key without retuning the instrument, and more easily play with other instruments. The disadvantage is that any given note may be up to 3% variant from it's proper frequency in a given key. Since tempered instruments have an even difference of the 12th root of 2 between the 12 note intervals of an octave.
We've got used to this modern convenience, and seldom notice the slight dissonance between "just" and "tempered" instruments. But in such a case as this lovely bit of Americana, we are reminded how much such pure traditional music can move the heart.
