Here is a zero-level precession photo of a monoclinic crystal:

Again, the 2-fold axis cannot be distinguished from a center of symmetry until upper level photos are obtained:


In this case, the 2-fold axis persists into the k = -1 and k = +1 levels. The intensity distributions in these two upper level photos are also identical - they are related by a mirror plane that is perpendicular to the b*-axis (parallel to the a*c*-plane.) Due to the 2/m Laue symmetry of this monoclinic crystal,