Experiments show that the excess or deficiency of diffracted intensity in the image of the first fringe in stacking-fault-type fringe patterns on X-ray projection topographs can give a clear indication of the sign of the fault vector even when μ t is no greater than unity (μ is the normal linear absorption coefficient and t is the crystal thickness) provided that the crystal structure is such as will exhibit fairly strong anomalous transmission (Borrmann effect) in the thick crystal case for the Bragg reflections used. A quite simple diffraction theory suffices to account for the observations. Sign determinations based on this theory agree with those expected at Brazil twin boundaries parallel to major rhombohedral planes in quartz.