Recently, Bragg and Williams have given an explanation of the variation of the degree of order in an alloy with temperature. According to this explanation it is essential to consider the ordering as a co-operative phenomenon characteristic for large assemblies of atoms. The force, V, tending to produce order, is, according to this view, a function of the degree of order, because in order to remove an atom from a “right” to a “wrong” position one requires more energy the greater the number of “right” atoms in the neighbourhood, i. e. , the greater the degree of order. One is thus led to consider two functions: V ( s ), describing the dependence of ordering force on order, and s (V), the order produced by a given ordering force, at given temperature. If both these functions are known, they implicitly define the degree of order as a function of the temperature.