Stacking fault contrast arises from the phase shift α = 2π ḡ · experienced by the diffracted wave at the fault. In an ordered L12 lattice, the displacement <111> is not necessarily equivalent to a displacement of the type <112>. 1 and 2 differ by a displacement <110> which can introduce a phase shift of π when ḡ is a superlattice diffraction vector. The physical difference can be seen by examining the formation of an intrinsic fault. When the fault is produced by removal of a close packed plane, <111>, no nearest neighbor violations of order result; on the other hand,nearest neighbor order is disrupted by shear of the <112> type.Similarly, there are two types of extrinsic faults. Values of α are listed in Table I for [111] and [112] with ḡ of the form 100, 200, and 300. As expected, the two representations are equivalent when ḡ is a fundamental reflection. For g = 100, 010 the values of α for 1 and 2 differ by π because of the antiphase boundary associated with the latter displacement.