Drifting Unisons

[Richard E. West]

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