A Hardanger fiddle (or in Norwegian: hardingfele) is a traditional stringed instrument used originally to play the music of Norway. In modern designs, this type of fiddle is very similar to the violin, though with eight or nine strings (rather than four as on a standard violin) and thinner wood. Four of the strings are strung and played like a violin, while the rest, aptly named understrings or sympathetic strings, resonate under the influence of the other four. (source)