Many fiddlers, particularly beginners only want to play out of the A or D position. It leaves room for a lot of the droning that defines the way Cajun fiddling sounds, and the D accordion is perfectly suited for that. Some of the more talented folks will stretch themselves and become proficient in the C and G positions enabling a standard tuned fiddle to play comfortably, with droning behind a C or D accordion. Those same people, when tuned down a whole step, can play behind a Bb or C.
Sadly, there are those who tune down two whole steps to play with a Bb. This results in floppy strings, scratchy tone and intonation problems. I know, personally, maybe five people who play standard tuned all the time and will attack pretty much any key. Two of them, can do it with no hesitation. Though the droning can't always be done, a fiddler's usefulness increases exponentially!
For Cajun music it doesn't usually come up, because most tunes are in C, G, D or A for the C and D accordions.
Bluegrassers play in Bb or B all the time, in standard tuning. The tendency is to use the index finger kind of like a capo, i.e. play the first two strings at the equivalent of the first (Bb) or second (B) "fret", which gives you the fifth of Bb-F or B-F#, then play with the other three fingers of the left hand above that "capo".
You can't get the unisons, but you can still play lots of double stops, and you can still play some open strings, like the open A will give you the flat 7th in B, etc.
Hi Nick, as fiddler i agree with Ganey & Ron, though i'm not too fond of B (and C#, G# F#). But Bb is not too bad, as are F and C, but some cajun stuff doesn't sound right without the proper (1st & 5th) open strings. Even some tunes in G sound better on a whole note lower tuned (FCGD, or GCGD or GDGD) tuned fiddle than a standard tuned one, especially the haunting slides up and down to the 3rd position. Problem is too: a good loud fiddle to tune below sea level is hard to find. Lucky I got one, a bit too fragile for steel strings, and 200 years old. Some one gave me as a real wreck. Happily heated up the old glue pot, fixed cracks and loose joints, strung it up and now sounds great even one whole note down. - nout
That's exactly how a mandolin player handles things in Bb or any key up the scale. Bar the 3rd and 4th string at the appropriate root. Violin and mando are tuned exactly the same.