Airport runways are "named" by number. The number designated for each runway is its magnetic bearing in degrees, divided by ten. For example, a runway which runs North to South (180°) would be 18. Obviously, this runway also could be used by an approach from the South, and coming from that direction the runway would be at 360°. To alleviate this confusion, runways are typically referred to by both numbers. 18/36 and 9/27 are common runway numbers as these would refer to a North/South runway or an East/West runway, respectively.

Many airports have more than one runway, sometimes ones which are parallel. When there are two parallel runways, each is given a "Right" or "Left" designation based on its position relative to the other. Take the following airport as an example:

                      ___
      +-+    +-+     /  /
      |B|    |C|    / D/
    +-+ +----+ +---+  /
    |A               /
    +-+ +----+ + -  /       /|\
      | |    | +/  /         |
      | |    |    /          N
      | |    |   /
      +-+    +-+

Here there are four runways, A, B, C, and D. A would be numbered 9/27, D would be 3/21, and B and C would each be 18/36 -- raising the problem of parallel runways. If approaching from the South, B would be 36L and C would be 36R. From the other direction, B would be 18R and C would be 18L. Hence, B is runway 18R/36L and C is runway 18L/36R.

The choice of which runway is being used and from which direction is primarily based on wind/weather conditions and other air/ground traffic.

Although the Left/Right designation is used in the case of parallel runs to avoid confusion, it is also used with single runs to inform pilots of which side the circuit is on.

Thus, in a pre-flight briefing, a pilot might be told that his runway is '25L with EFATOs to the right.' This means that his runway is runway 25, at 250 degrees (as explained above), the circuit is an anticlockwise one, and if he has an engine failure, he is to head right. Any aircraft on a right-handed circuit will be known to be having problems and so can be given priority.

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