In the diagram on the left a Bragg reflection occurs from the 101 plane. If the crystal is rotated 180° about the horizontal direction, the reflection occurs on the “other” side" of the 101 plane (right diagram). This amounts to a reflection from the plane.

The intensity of a reflection is determined by the amount of electron density in the Bragg plane; certainly this will be the same whether the reflection occurs from the “front side” of the plane or from the “back side” of the plane. Thus, in general, all diffraction patterns are expected to be centrosymmetric and it will always be true that


This relationship expressing the inherent centrosymmetry of X-ray intensities is known as Friedel's Law.