There are five different major groups of systems that measure the 6DOF for the Virtual World. These groups are called the Tracker Technology systems. The first of the five systems is a mechanical device that uses rigid rods to measure the angels of a joint and transfers the information to a computer. The second system sets up an electromagnetic field around an object that calculates the position of receivers placed on certain reference points by measuring the distance from each other. Another system uses an infra-red light direct towards an object where the distance is calculated. The fourth system uses sound that is sent as a signal to microphones placed at points on an object, then the time is recorded from the moment the sound is made to the time when the microphones receive. Form the difference in time the distances can be calculated. The last system uses gyroscopes to measure changes in rotation about 1, 2 or 3 axis. Though none of these systems has been proven to be completely accurate there is certain to be some sort of lag or distortion in the Virtual World.