de Valcourt (1966) lists 11 different measures of axial symmetry, of which the one described here is number three. He requires each such measure to be invariant under similarity transformations of the given shape, to take the value one for symmetric shapes, and to take a value between zero and one for other shapes. Other symmetry measures with these properties include the ratio of the area of the shape to its smallest enclosing symmetric superset, and the analogous ratios of perimeters.