The scratches produced by single abrasive grains in overcut fly milling show that the transverse shape of a grain is closely approximated by an arc of a circle. This radius of curvature is found to be independent of grain type and grinding conditions but varies with the grain size. The equation for undeformed chip thickness for surface grinding is rederived in terms of this radius. The important role that the transverse curvature of the grain plays relative to surface finish is also discussed.