Abstract: Compensation is made for inherent lateral shifting of portions of a pivoted working surface along a work surface, when the pivot point of the working surface is maintained on a line of feed into contact with said work surface, during relative pivoting of said working surface into conformity with said work surface. The working surface is supported by a quadrilateral linkage system having a remote instantaneous center point which, during pivoting of the working surface, is maintained substantially on a quiescent centerline of the linkage system. By spacing the pivot point of the working surface from this center point along the centerline, the pivot point is caused to move laterally of the centerline during exercising of the linkage system. The distance and direction of spacing the pivot point from the center point is selectable so as to cancel the inherent lateral shifting referred above.