# Pseudo symmetry

A crystal space can in general be divided in *N* components S_{1} to S_{N}. When a coincidence operation φ(S_{i})→S_{j} brings the *i*-th component S_{i} to coincide with the *j*-th component S_{j}, for any *i* and *j*, φ is a symmetry operation of the space group.

Sometimes, φ brings S_{i} close to, but not exactly on, the position and orientation of S_{j}: in this case the operation mapping S_{i} onto S_{j} is not crystallographic but the linear and/or rotational deviation from a space group operation is limited. For this reason, it is preferable to describe the crystallographic operation φ as a **pseudo symmetry operation**.

Pseudo symmetry operations for the lattice play an important role in twinning, namely in the case of twinning by pseudomerohedry and twinning by reticular pseudomerohedry.