Chicago’s O’Hare International Airport has sixteen different runways (keep reading, that actually means eight, of which five are used). The two most used runways are 27L (landings) and 28R (take-offs). Runways are named based on their direction on a scale of 1-36, which, when adding a zero to the end, designate the compass direction of the runway as you look down from the beginning to the end.
The compass is a 360 degree marking of a circle, with 90 degrees facing East, 180 degrees facing South, 270 degrees facing west, and 360 degrees facing North. For example, a runway marked 27, faces the compass direction 270, which is West. A runway marked 5, faces 50 degrees or Northeast (the runway might be facing 45 degrees, but is rounded to 5, so as not to use 4.5).
The direction you are facing matters. If you are looking down the runway from the South end of runway 36, you are facing due North. However, if you were at the other end of the same runway, but facing the other direction, the same runway would be marked 18, as it is also facing due south. In this way, not only are you able to tell which runway you are on, but which direction you should use it. So, runway 27 will be marked runway 9 on the other end, and runway 5 will be marked runway 23 on the other end.
Finally, most major airports will have parallel runways, so L-C-R designations are used for left, center, and right. At O’Hare, the majority of landings occur on runways 27L, 27C, and 27R. These three parallel runways face west and allow places from the East Coast to land without turning around — three-at-a-time.
As you may have surmised, facing the other direction, the runways have the opposite designations. 27L is 9R, 27R and 9L, and the center runway, 27C, is 9C.