Whenever an air traffic controller is posted to a new unit or starts work on a new sector within a particular unit, they must undergo a period of training regarding the procedures peculiar to that particular unit and/or sector. The majority of this training is done in a live position controlling real aircraft and is termed On the Job Training (OJT), with a fully qualified and trained mentor or On the Job Training Instructor (OJTI) also 'plugged into' the position to give guidance and ready to take over in a second should it become necessary. The length of this phase of training varies from a matter of months to years, depending on the complexity of the sector.