Observation of a slip gauge on a flatness interferometer produced fringe counts numbering $$10$$ and $$14$$ for two readings. The second reading is taken by rotating the setup by $${180^ \circ }$$ Assume that both faces of the slip gauge are flat and the wavelength of the radiation is $$0.5086$$ $$\mu m.$$ The parallelism error (in $$\mu m.$$) between the two faces of the slip gauge is