Uppland is the centre for the Nyckelharpa-culture but there are evidence that the instrument has been used in most of southern Sweden, possibly even in Norway. Nyckelharpa is a very old instrument and there are paintings of it from the 15th century. Most of the organologists who have studied the instrument have supposed that the nyckelharpa has had sympathetic strings from the beginning. However the oldest preserved nyckelharpa, the so called moraharpa (found in Mora), has no sympathetic strings. It is not possible to say whether or not the nyckelharpa was important for the use of sympathetic strings on violins. It is also possible that both violins and the nyckelharpa was equipped with sympathetic strings as a result of the european trend with instruments such as the viola d'amore. Given the current hypothesis that Swedish fiddles with sympathetic strings originated in Skåne, it does not seem probable that the nyckelharpa was important for the evolvment of the double-deckers and other understringed fiddles.
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
According to the swedish instrument collector and hobby scientist Daniel Fryklund a Viola d'amore is an alto viola da braccio with five to seven melody strings and usually the equal amount of sympathetic strings. The instrument is a transitional form between Viols and the instruments in the violin family. Some features that are inherited from the Viols are:
The biggest difference between the Viola d'amore and the Viols is that the latter is held similar to the cello (in the lap) while the first is held as a violin. Viols are also freted while the viola d'amore is usually not (even though there are examples). The oldest Viola d'amores are from the late 17:th century but it reached its popularity peak during the 18:th century. There are quite a lot of classical music composed for the Viola d'amore, for instance by such composers as Bach, Telemann, Vivaldi and Leopold Mozart (Wolfgang Amadeus father). The instrument was tuned in a way which suited the tune, so called scordatura. Viola d'amore was however not only used in fashionable contexts. Fryklund writes that it was widely used during the 18:th century, even by amateurs and women (a fact which doesn't seem strange at all today but maybe was in the first part of the 20:th century) but the popularity decreased during the 19:th century. The viola d'amore had a renaissance during the late 19:th century but was never as popular as 100 years earlier. In the last decades enthusiasts have discovered the instrument, both folk musicians and classical violinists. Scordatura has always been common in many folk music traditions but is never used in classical music except by these viola d'amore players