Another drawback of this technique is that the screen showing the interface has to be in front of the user and might thus block visibility of the trajectory to be followed in guidance applications. This makes it necessary for users to move their head (some of whom, with certain types of injury, are unable to do so) and it also means that the position of the head vis-à-vis the interface is lost, thus upsetting the calibration of the system. Fig. 4(a) shows the user guidance interface. Commands should be as big as possible with no overlapping, in the interests of the best selection thereof; there should also be certain safety areas that make it impossible to select one command when another is desired.