this thread made me think of this:
http://en.wikipedia.org/wiki/Eddie_Van_Halen#Tuning
Tuning
Though rarely discussed, one of the most distinctive aspects of Van Halen's sound was Eddie Van Halen's tuning of the guitar. Before Van Halen, most distorted, metal-oriented rock consciously avoided the use of the major third interval in guitar chords, creating instead the signature power chord of the genre. When run through a distorted amplifier, the rapid beating of the major third on a conventionally tuned guitar is distracting and somewhat dissonant.
Van Halen developed a technique of flattening his B string slightly so that the interval between the open G and B reaches a justly intonated, beatless third. This consonant third was almost unheard of in distorted-guitar rock and allowed Van Halen to use major chords in a way that mixed classic hard rock power with "happy" pop. The effect is pronounced on songs such as "Runnin' With the Devil", "Unchained", and "Where Have All the Good Times Gone?".
With the B string flattened the correct amount, chords in some positions on the guitar have more justly intonated thirds, but in other positions the flat B string creates out-of-tune intervals. As Eddie once remarked to Guitar Player:
“A guitar is just theoretically built wrong. Each string is an interval of fourths, and then the B string is off. Theoretically, that's not right. If you tune an open A chord in the first position and it's perfectly in tune, and then you hit a barre chord an octave higher, it's out of tune. The B string is always a cotton-headed ninny to keep in tune all the time! So I have to retune for certain songs. And when I use the Floyd onstage, I have to unclamp it and do it real quick. But with a standard-vibrato guitar, I can tune it while I'm playing.''[21]