An aircraft taking off will usually be expected to follow the circuit in use, and one arriving at the field to land will be expected to join the circuit in an orderly fashion before landing. This is often accomplished using an overhead join, or by entering on any of the circuit legs, traffic permitting. Aircraft are expected to join and leave the circuit in an orderly and safe manner. Sometimes this will be at the discretion of the pilot, at other times he will be directed by air traffic control. Airfield operators will set up circuits according to the runway in use, the local conventions for noise abatement or other ground features, and the current usage of the airfield. While many operate a completely standard pattern, in many cases it will be modified according to need.
Circuits can be defined as left or right hand, according to which way the turns in the circuit lie. Circuits are typically rectangular in basic shape, and include the runway along one long side of the rectangle. Each leg of the circuit has a particular name:
Military airfields often dispense with the crosswind and base legs, but rather fly these as circular arcs directly joining the upwind and downwind sections.
An airfield will define a circuit height, that is, a nominal altitude above the field (see QFE) that pilots are required to fly at while in the circuit. A typical circuit height would be 1000ft.
The use of a circuit at airfields is for safety - rather than have aircraft flying around the field in a haphazard fashion, by using a circuit pilots will know where to expect other air traffic from, and be able to see it and avoid it. GA pilots flying under VFR conditions will not be separated by air traffic control, and so the circuit is a vital way to keep things orderly.
A pilot undergoing training will often fly many, many circuits one after another. Usually, each landing is followed immediately by a take off and further circuit. This is called a touch and go.
Americans usually refer to the circuit as the traffic pattern or simply pattern. Similarly, circuit height would be referred to as pattern altitude.