in order to cut a key duplicate, it is necessary to measure the original key very precisely. there is a great variety in the key measuring devices currently in use; however, they follow the common basic principle of using a known light source of some kind and measuring the features of the light reflected off the key. in some variations of these systems, only a small part of the key can be measured at one time, and therefore some part of the system needs to be physically mobile to measure the entire key; be it the key itself, the light source or the camera measuring the reflected light. most of the systems also require the key to be positioned exactly straight in the device in order to be measured, and even a slight deviation means that the blank key will be cut wrong and will be unusable.