 In practical terms, the plate is inserted between the perpendicular polarizers at an angle of 45 degrees.This allows two different procedures to be carried out to investigate the mineral under the crosshairs of the microscope.More simply, in ordinary cross polarized light, the plate can be used to distinguish the orientation of the optical indicatrix relative to crystal elongation – that is, whether the mineral is "length slow" or "length fast" – based on whether the visible interference colors increase or decrease by one order when the plate is added.