It was shown by Paterson that in a cold�worked face-centred cubic metal, deformation faults produce a peak shift, and twin faults produce a peak asymmetry. If the peak shape is represented by a Fourier series, the asymmetry is expressed by the sine terms. However, the evaluation of the sine coefficients is difficult, due to overlapping of the tails of neighbouring refleotions and uncertainty in the peak origin. In this paper, a method is developed which combines the tails of the (Ill) and (200) reflections. This allows a determination of the twin fault probability in face-centred cubic metals which is more nearly independent of overlapping of the tails and choice of the peak origin.