I've noticed the drifting unison phenomenon for years and origianlly thought it was due to bridge movement either vertically or the dreaded bridge rolling. Then the varying-string-length argument came along and that seemed to help answer the question but I never thought it was the complete solution. Since Fred brought up the question again, I looked at the bridges and agree with whomever it was that thought it's the notching that somehow affects unisons. The bridge pins of the center strings are aligned closer to the center line running the length of the bridge than are the two outside strings. Therefore if there's any bridge movement at all the center strings' bridge pins may not move much in relation to each other, but the two outside strings would tend to move in opposite ways because of the position of the pins in the notch created by chiseling out the bridge. One possible proof of this is that where the notching is most pronounced, i.e. the lower tenor area of the long bridge, the effect is greater. Richard West
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