Abstract
In a long period ordered alloy, interference between superlattice and satellite beams allows to obtain fringe systems, the characteristics of which are analysed within the two beam kinematical theory of electron diffraction. Expressions for the intensity are given in the different cases and fit with electron microscopy observations performed on an Ag3Mg alloy of 22 at% Mg composition. These observations show conclusively that antiphase boundaries occur regularly on every second (100) plane for such a concentration.