Summary: Single-crystal diffraction patterns produced by transmitted radiation, both X-rays and electrons, reveal varying degrees of disorder and long-range order in common phyllosilicates from several different rock types. The transmission electron micrographs and their selected-area diffraction patterns demonstrate the presence of numerous stacking faults parallel to (001) of muscovite and chlorite. Individual stacking faults can be recognized by the diffraction-contrast fringe patterns they cause, and partial dislocations can be seen where such faults terminate inside a crystal. Long-range order of muscovite explains what seemed to be spurious, high ‘background’ levels that are sometimes encountered in the analysis of rock fabrics by transmitted X-rays.