Mark - I've been playing with various old time fiddlers in our district off and on for about 20 years, and it doesn't get any easier!

You seem to have the basics well in hand. I tend to try to identify common patterns, and if you can quickly hear the similarity in structure and say to yourself "well that's like Soldier's Joy, except in C," you can squeak by. A lot of the old time waltzs use the circle of fifths, or "five-chord turn around," or "Sears Roebuck turn" as one old guy called them - (G,E,A,D,G)

Many of the older reels can be played with only two chords in the A part, and then they add the third chord in the B part. Some of the southern tunes may sound like they require a minor, but a dyed-in-the-wool "old timey" fiddler prefers a major chord! "Waiting for the Federals" aka "Seneca Square Dance" in key of G is one of those. My ear hears an Em in there, but fiddlers (like Bruce Molesky) would rather you play a C.

Some fiddlers play real clean and you can hear their "changes" easily. Others can be pretty broad in both their intonation and tempo, and you can just barely keep it between the ditches. A fiddler who plays with a lot of double stops can also make you wonder which key they're in at times. Probably one way to know you're playing the right chords is when the fiddler quits rolling his/her eyes and grimacing! :}

One trick I learned from another guitarist is how to play with a fiddler who can't hold tempo. We had a nice old guy, now deceased, who had beautiful tone, but thought "Time" was a magazine and he'd forgotten to renew the subscription! My guitarist friend abandoned the standard "boom-chuck"(bass note-chord strum)style and went to a windshield wiper pattern of strumming up and down (sort of like Celtic guitarists) with no perceived pulse or beat, just a wall of sound coming out of the guitar. When the fiddler's tune required a chord change the guitarist would finger the new chord, meter be damned. It worked, and he could get through a set of tunes with this fiddler.