- "Standard" tuning -- the notes most classical violinists tune their strings to -- is mostly just used for tunes in G or C.
- For D tunes, they'll raise the low G to an A.
- For A tunes, they tune to AEAE.
Fiddlers carrying around twin-fiddle cases will often have one fiddle in that case tuned to one key and the other tuned to a second.
If you're a fiddler, cross-tuning brings you several advantages:
- It gives you convenient drones on neighboring strings, so you can play more double stops more easily.
- It turns some of your strings into sympathetic strings, that vibrate even when you don't play them, which makes the sound richer.
- You can play unison double stops with your third finger, which is much stronger than your fourth.
- A tunes can be played with the same fingerings in two octaves.
- Tunes and tunings co-evolve. D tunes have the notes they do because those notes are easy to play on a fiddle tuned for that key.
Mind you, this isn't just for Appalachian music or just for fiddles. If you're a classical violinist, this is scordatura. If you play backup guitar for Irish music, you may have used tunings like DADGAD (pronounced "dad-gad") or dropped-D, and a clawhammer banjo player uses far more tunings than any fiddler.
I don't retune my mandolin; mandolin string tension is so high that I worry I'd snap a string, rip off my bridge, or pry my top loose. Still, I, too, am a big fan of fiddlers' retuning. Jams often stay in one key all night long, so I can practice scales, arpeggios, and even chords in a single key for hours.
