It's not what you think. I've seen jigs for working on basses at Stewart MacDonald and one of the technicians that used to work on my basses did all action adjustments with the bass in a playing position. However, nearly every other technician I have ever seen makes adjustments with the bass lying flat. The justification said technician gave was that gravity has a definite effect on action adjustments and one can be more precise by holding the bass in the natural playing position. That makes sense to me, but are we talking about thousandths of inches here? Is this taking it to the nth degree or something that makes a real difference? Thanks, -Eric.