I've a friend who's a very good guitar player ( been playing 40+ years, much better with by ear stuff than I am) and who insists that when putting several tunes together into a medley, at least 2 of the tunes must be in different keys.
I have no idea if this is generally true or just an opinion on his part. It's easy enough for me to do if I'm in G, that is change keys, but it's a bit tough in something like capoed double D (C) tuning, from which (for me) it's a real challenge to play something in say, G or D.
I've entertained the idea of d-tuners as a way around this and posted about that, but my enthusiasm for that idea has diminished. Too complicated.
Any opinions out in OT BHO land on this?