As with aerodrome control and approach control, so with area control, coordination is the vital function which links all of these services. Aircraft which are receiving an air traffic control service or an advisory service from an air traffic control center (ATCC) or from a sector within that ATCC, must not be permitted to penetrate the airspace of another ATCC or sector unless prior coordination has taken place. In all coordination it is important to recognize that they must take place ahead of the concerned aircraft＇s movement, and that the responsibility for initiating this action rests with the controller of the unit or sector which is transferring control. It is further a requirement of coordination that the transferring controller must comply with any conditions specified by the accepting controller. For example, the accepting controller may require the aircraft to go to a higher or lower level or require the aircraft to be delayed, due to traffic in his sector. The reason, quite simply, is to ensure that no aircraft is transferred from one controlling authority to another until the airspace is clear to receive it, in accordance with the standards of separation previously described. The steps in coordination, which take place progressively, can be described as notification, negotiation and agreement. How this process is achieved is the subject of local agreement between the units concerned, often including neighboring foreign states. There are, of course, occa sions where it is not necessary to coordinate each individual movement, but these instances are subject to detailed operating procedures, agreed to and implemented by both parties. In regard to coordination with aerodrome control and approach control, the role of area control is that of a parent body, whose task is to ensure the overall efficiency and safe operation of the air traffic services. Approach control and aerodrome control units are required to comply with instructions which area control issues to achieve the objective of the air traffic control service, throughout the specific airspace for which they are responsible. For exam ple, when it is necessary to coordinate the departures from one or more aerodromes, the time at which individual aircraft can take off is specified by area control. Similarly, with traffic which is inbound to aerodromes, it is area control who issues the ATC clearance to the air craft, either to proceed to a holding facility serving the aerodrome or, if the arrival flow into a particular aerodrome permits, clear the aircraft to make R/T contact direct with approach control.1. Coordination is very important in ATC because .