I guess I either know too much or too little. I don't see any usefulness in the circle of fifths. What's it do for the practical musician?

josepp, this is folk music forum. If you don't like folk music, why are you here? There are HUGE differences between many types of folk music an classical music, and many of the classical rules simply don't apply to folk.

Off topic, but it was mentioned earlier: The nyckelharpa players I play with don't use equal temperment. They carefully tune specific strings a certain number of cents flat or sharp in order to maximize the intonation for the common nyckelharpa keys. Somewhere in between equal temperment and just intonation.