Tempering, continued...
I was watching a Tony Rice video in which he talked about temper tuning guitars. (He uses a simple tuning fork which he pulls out of a pocket then tunes an A on his 3rd string, if I remember correctly, tuning outwards from there). He said that not only is tempering necessary for guitars (as well as banjos), but it is also sometimes necessary to tune for a particular song. His new signature song is "Shenandoah" played in open EADGBE tuning but in the key of D. He says that he will change the tuning on a few strings to compensate for this tune which operates more around D chords than G chords. When I play "Doug's Tune" on banjo, I drop the 3rd string a little from my G tuning tempering because the song revolves around a D chord.
More than you could ever hope to understand much less want to know about temper tuning.