A couple of folks have brought up the truly annoying problem of keeping B strings in tune on both banjo and guitar. I had a chance to discuss this with someone several years back who understood the problem well (I forget who - but it was someone in a position to speak with authority). It was explained thusly:

The problem stems from the way Western music defines the pitch B. If you go through the various naturally occurring harmonics by dividing a vibrating string into its multiples (halves, thirds, fourths...) you will eventually come across all of the tones of our chromatic scale, *but* the pitch we define as B is reconciled slightly from its natural relation to the harmonic series in order to 'harmonize' with our scale system.

Hence, B is always a tough pitch to tune as it never really *IS* in tune. I have also been told that this is why the traditional violin family of instruments avoided the pitch as does the wind family.

Perhaps someone with a better understanding of this can offer a more cogent explanation. =)

Thanks to John Lawless for this info!