 "End standards", that is standards whose length is defined as the distance between their flat ends such as long gauge blocks or the , must be supported at the Airy points so that their length is well-defined; if the ends are not parallel, the measurement uncertainty is increased because the length depends on which part of the end is measured.For this reason, the Airy points are commonly identified by inscribed marks or lines.For example, a 1000 mm length gauge would have an Airy point separation of 577.4 mm.