The Bessel points are located 0.5594 of the length of the rod apart, slightly closer than the Airy points. Because line standards invariably extend beyond the lines marked on them, the optimal support points depend on both the overall length and the length to be measured. The latter is the quantity to be maximized, requiring a more complex calculation. For example, the 1927–1960 definition of the metre specified that the International Prototype Metre bar was to be measured while "supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other." Those would be the Bessel points of a beam 1020 mm long. Other sets of support points, even closer than the Bessel points, which may be wanted in some applications are: