The hardingfele or hardanger fiddle is the national instrument of Norway and is still very common in norwegian folk music. A couple of things makes it different from a standard violin:
The origin of the instrument is not proved and there are two hypothesises. Either the sympathetic strings was attached to an older norwegian type of bowed instrument as a result of the viola d'amore-trend in Europe or else is the origin norwegian. Research is in progress in Norway and it might give clues about the origin of swedish fiddles with sympathetic string as well.
For further information, see wikipedia
If you have information about fiddles with sympathetic strings that you like to share with me please email info [at] resoneramera [dot] se
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