Each fixture must have one secondary reference point, either a Secondary Liner Bushing or a Slotted Locator Bushing. Secondary Liners have an extra .001" ID clearance (+.0012/+.0017" tolerance) to avoid binding. Slotted Locator Bushings have a tight ID tolerance in one direction and full relief in the perpendicular direction. Slotted Locator Bushings provide better accuracy than Secondary Liners, but take a few extra steps to install, and are not available in all sizes. Note: a Primary Liner could theoretically be used as a secondary reference if hole locations are held to +/-.0002".