This bass must surely take the prize for having the most names, some accurate, some not — some just plain silly: bass, bass viol, contrabass, string bass, double bass, bass violin, upright bass, bass fiddle, and my personal least favorite, bull fiddle — ignoring entirely the electric bass and its several variations. And of all those, it seems that bass violin may be the least accurate of these names, for the bass is not a member of the violin family at all, but rather, the last remaining member of the viol family in the conventional orchestra. The violin, viola and cello all have a very important characteristic in common. They are tuned in fifths. The bass is tuned in fourths like its viol family forebears. Its most common shape has sloped, rather than rounded, shoulders, gamba-style corners and often a flat back – all common characteristics of the viols rather than the violins.
What does all of this have to do with gypsy jazz? Second only to the fiddle, its probably the most important instrument in your band.