scrubber wrote: There are, of course, many other examples. I've found that sometimes the 'unsual' keys exhibit some idiomatic characteristic of the fiddle that becomes lost when an attempt is made to put the tune into a 'normal' Bluegrass key. Ultimately, the key that makes the instrument '"shine" is probably the best choice, even if it
is a bit unusual......

Tunes tend to be done in the best key for the instrument that drives them.
Heavily banjo-driven tunes tend to be done in "G", "A", or any other key that's reachable with a capo and will let the banjo player use what he knows in "G".
If you notice, guitar-driven tunes (and there are few of those in Bluegrass) tend to be done in "C", or capoed-up from "C" because that's a good guitar key.
Some of the guys who do really hot crosspicking can't do a large number of songs in anything other than "C", or "C" capoed-up, because the notes just aren't reachable unless you're in "C".
You'll even see Tony Rice doing that a lot. He's always tended to capo at the third fret or second fret and play out of "C" formations, and least when he was doing Bluegrass.
Mother Maybelle always played "Wildwood Flower" in the key of "G", but she did it way up the neck with a capo, so she could be actually be playing out of chord-formations in "C".
And fiddle... well because of how the strings are arranged, and because so much Bluegrass/Old Time never much gets out of first position, "D" and "A" are the easiest keys for a fiddler to play in.
You have open strings within the scale on both sides of the root-note string, not only for droning against, but you have lots of useable notes available for melody and double-stops on both of those other strings without getting out of First Position.
Also, a song which is in either of those keys can often be played in the other key by simply moving over to a different starting string, using all the same fingering.
That tends to put fiddle-driven tunes in one of those two keys pretty often. I have the impression that more fiddle tunes done (and notated) in "A" than any other key, when it comes to Bluegrass, Old Time, and Irish fiddle.
The key of "G" probably comes in at third place.
You can start your "G" scale from the "G" note on the "D" string, and still have it not be too dificult, though you have fewer open notes, making it marginally more difficult that "A" and "D".