The GLS (Global Least Square) superimposition uses a least-square criterion to fit one figure ontoanother. It is therefore a global fit that involves all landmarks. After superimposition, the sum of differences between corresponding landmarks is minimal.

The figure below illustrates this fitting. In this case, the head film tracings of pre treatment and post treatment of a skeletal CI3 patient showing an anterior crossbite have been fitted one upon another. Major differences between the landmarks of these two sets of lines are located on incisors, which corresponds to the anatomical points mostly displaced by treatment.

This GLS fitting using all landmarks minimizes the bias introduced by the superimposition on a single variable plane. Decades of researches showed that it is impossible to find an invariant anatomical plane

Due to this natural variability and fitting method, we consider that differences in structures that are not accessible for treatment must not be taken into account.