Aircraft may arrive at the landing site from any direction, so a safe means of integrating into existing traffic and aligning with the runway is required. The overhead join is the standard method as used by most general aviation aircraft flying under the Visual Flight Rules.
Prior to arrival, the pilot will liaise with air traffic control over the radio to establish the runway in use, the circuit height and direction (left or right hand), the QFE of the field. If the airfield is not equipped with ATC or radio, he will need to establish these things before arrival by contacting the airfield prior to departure. Once the pilot knows these things, he is able to perform a standard overhead join. This involves positioning the aircraft so that it is flying in the general direction of the circuit, but at some height above it - usually an extra 200 feet or so. He overflies the runway, visually looking for other traffic in the circuit, and descends to circuit height on the dead side (opposite that of the circuit). He can then safely position himself in the circuit behind or between other traffic without conflicting, and land in turn.
Alternatives to the overhead join are to directly join the circuit by entering at a suitable corner, or a straight-in approach. ATC and the pilot will negotiate which is the most appropriate method according to the current traffic situation.
At airports where there is no air traffic control, a common procedure is to enter the circuit at a 45-degree angle to the downwind leg. An aircraft entering using this method would start below and to the right of the picture above and aim for the middle of the runway. When the aircraft is about to cross the downwind leg, the pilot makes a left 45-degree turn to enter the pattern in the middle of the downwind leg.
If a pilot approaching an uncontrolled airport does not know which way the wind is blowing, and therefore which runway to use, he may first overfly the field to view the wind sock. This is generally done at an altitude safely above the circuit and often followed by a descent and a 45-degree downwind entry.