Displayed below is a zero-level precession photograph of a triclinic crystal. According to Friedel's Law, all X-ray photos are centrosymmetric whether they describe a centrosymmetric crystal structure or not. At this point, with only a zero-level picture, it's not possible to distinguish a center of symmetry from a 2-fold axis. The reciprocal lattice shown could be the a*c* plane of a monoclinic crystal with the 2-fold axis parallel to b* (perpendicular to a*c*).

The two photos below are the k = +1 and k = -1 levels. The possible 2-fold axis does not appear on these upper level precession pictures; the only symmetry element present is the center of symmetry which causes intensities on the +1 level to be related to intensities on the -1 level.

Due to the center of symmetry,