‘Fiddle’ is actually the generic name for all bowed stringed musical instruments, so including the violin, and the ‘Cambridge Companion to the Violin’ (*), learns us that in fact, the instruments are totally identical, and that it’s just the style of playing and repertory in which it is used are different. This view is pretty questionable, though.
Fiddles are violins that are used in country, bluegrass, or folk music, whereas violins are used to play jazz or classical music. Fiddles are also used, though, to play a number of other music styles such as Mediterranean and Middle Eastern music, and Western swing. Note also that the term ‘violinist’ is additionally used identify classically trained violin players, and the term ‘fiddlers’ for musicians who’re playing local dance music.
There are quite a few people who believe that there is a considerable difference between fiddles and violins. The Boise, Idaho, based company Telford & Sons Violins (trade and teaching), is stating that fiddles can be produced by any Grandpa from an old barn door or a fencepost, but that violins are crafted in line with accepted methods and have specific proportions, and are created from from traditional materials.
Usually, fiddles are also adjusted a little differently than violins. There are fiddle players, for example, who are preferring that their instruments have strings in a lower-than-usual position and that the bridge arch is flatter. Fiddle players usually also prefer the use of louder strings, and some have come up with variations of the traditional 4-stringed violin, for example a 5-string or an amplified instrument. There are many well-known and famed fiddle players who say that the violin sings, but that the fiddle dances.
(*) The Cambridge Companion to the Violin is offering performers, scholars, and students a composite and fascinating survey of the repertory and history of the fascinating instrument, from its origins, through the centuries, to the modern day. The Cambridge Companion to the Violin contains 15 essays, produced by a highly skilled group specialists, and aims to develop a broad historical perspective of the violin from every possible relevant angle.
The most important issues and subjects addressed include the structure and development of the violin, the fundamental acoustic properties of the instrument, it’s principal exponents, the teaching principles and techniques, ensemble and solo repertory, all sorts of pedagogical literature, various traditions in country and folk music, jazz applications and techniques, and all sorts of aspects of performing practice through the centuries.
The text is widely supported by diagrams, illustrations, and music examples, there is a useful appendix, a glossary of various technical terms, and a very extensive and interesting bibliography.