# Friedel pair

The couple of reflections h, k, l and ${\bar h}, {\bar k}, {\bar l}$ is called a Friedel pair, or Bijvoet pair. Their intensities are equal if there is no absorption, but differ otherwise. Friedel's law then does not hold. Generally speaking, when absorption is present, equivalent reflections generated by the symmetry elements in the crystal have intensities different from those of equivalent reflections generated by the introduction of an additional inversion centre in normal scattering.