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.