The effect on the x‐ray diffraction pattern of deformation faults and twin faults occurring at random on the (211) planes of a b.c.c. lattice has been investigated. In single crystal reflections, deformation faults produce peak shifts and peak broadening, and twin faults produce a peak asymmetry and peak broadening. For a powder pattern it is necessary to average over the different components of a powder‐pattern peak; the peak shifts average out to zero, and the peak asymmetries to a quantity which is generally too small to measure. Both types of fault produce a line broadening which varies strongly from one powder peak to another. Thehkldependence of the broadening is the same for the two kinds of faults, and only a combination of the two probabilities can be measured. Measurements were made on samples of cold‐worked &bgr;‐brass filings in different states of annealing. There were no measurable peak shifts or peak asymmetries. The ratios of the effective particle size values could be explained only if a major part of the particle size broadening is due to faulting. The results are discussed in connection with measurements on other cold‐worked b.c.c. metals.