In many traditions of folk music, the tunes are not written but are memorized by successive generations of musicians and passed on in both informal and formal contexts.
When played as a folk instrument, the violin is ordinarily referred to in English as a fiddle.
One very slight difference between fiddle and violin occurs in American (e.g., bluegrass and old-time music) fiddling: the bridge is shaved down so that it is less curved. This makes it easier to play chords.
Most musicians agree that the technical difference between a violin and a fiddle is the bridge. Most classical violinists prefer rounded bridges that allow them to more easily articulate the notes which have better clarity. Fiddlers often prefer flatter bridges that allow the playing of double notes and shuffles. In practice, most instruments are constructed with a rounded bridge to better accommodate the shape of the fingerboard.
Historically, the word fiddle also referred to a predecessor of today's violin. Like the violin, it tended to have 4 strings, but came in a variety of shapes and sizes. Another series of instruments which contributed to the development of the modern fiddle was the viol, which was played while held between the legs, and has a fretted fingerboard.