Coordination is an extremely important yet often forgotten factor in aviation and working as an Air Traffic Controller. This document will show you the basics of coordination and why it is crucial.
Imagine a flight from Warsaw to Munich. The aircraft stands at its gate in Warsaw. Firstly the pilot will get an ATC Clearance from the Delivery controller. Then DEL Controller will hand off the aircraft to the Ground Controller for push and taxi to the RWY. Prior to reaching the holding point, the aircraft will be handed off to the Tower Controller to obtain Take Off clearance. After lift-off, the pilot will switch to Approach / Departure Controller for the initial climb out. The APP will then hand off the aeroplane to Warszawa Area Controller for the final climb and cruise level portion of the flight. Few minutes before reaching the border of our country, Warszawa will handoff our flight to Praha ACC Sector, and then last but not least, Praha will issue a handoff to Munich ACC where the "flow" reverses: Munich Area Controller to Munich Approach Controller, Approach Controller to Tower Controller and Tower Controller to Ground / Apron Controller.
(Bear in mind that there might and usually will be more than just one sector of each of these (APPs/ACCs or even GNDs)).
As you can see, this is pure teamwork between everyone involved, and good coordination plays a key role here.
During every flight an airplane receives a lot of information and clearances from ATC. It is vitally important that none of these surprises the next controller involved and that the pilot does not receive two conflicting clearances from two different controllers.
Usually, given clearances fall under predetermined local procedures and agreements between different sectors, so there is no strong need to coordinate every detail. Although very often, there are situations that will need coordinating.
You don't need to look very far: issuing an ATC Clearance for a non-standard departure. A Delivery Controller will need to call Approach Controller and ask what he wants for the aircraft after take-off.
Looking ahead and predicting traffic situation in the next minutes is what ATCO do on a daily basis, and it's an essential part of their job. Therefore coordination comes in here very handy.
DEL<>GND<>TWR will be TWR<>DEL coordinating VFR clearances, issuing non-standard IFR departure clearances, coordinating aircraft on the movement area with those landing/departing. These are just a few examples to give you a broader context about our topic.
TWR<>APP<>ACC will be for instance TWR and APP coordinating more spacing between 2 arriving traffic to put 1 departure in between them. Or DCT for 500NM from Amsterdam to Warsaw agreed and coordinated between 5 ACCs. An Area Controller might call another Area Controller from an adjacent sector to request a certain aircraft on a certain (different) level during handoff. These and many many more are a day-to-day practice of all controllers. They get hundreds of telephones daily from another sectors with requests regarding coordination.
It is a possibility and good habit to make a short briefing with adjacent ATCs (especially when expecting high peaks of traffic flow, or before an exam, training or event) concerning possible:
Nonetheless, any briefing shall not be an exemption from coordinating because it is not impossible to predict all hypothetical situations.
As you can see, good and clear coordination is essential for flight safety and all involved controllers so that everybody knows what the aircraft is currently doing and what it is about to do.