A runway is a straight line—180-degrees—so each end is named for their clockwise heading on a 360-degree compass. For example, Runway 27 faces 270 degrees on a compass—due west. The opposite end of that same strip is called Runway 9, as it faces 90 degrees—due east. In addition, of each set of parallel runways will be named L (left), R (right) or C (center) depending on their heading. So a pair of parallel runways will be 9L/27R and 9R/27L—pronounced niner left, two-seven right—with the smaller number always mentioned first. A standalone runway will not have a left/right/center designator.

No more than three parallel runways on the same side of a terminal will share names. If there are four, at least one of those will be named one degree off from the others, with the number increasing north to south, east to west (examples: 8L/26R – 8R/26L – terminal – 9/27 at Bush Intercontinental, or 9L/27R – 9C/27C – 9R/27L – terminal – 10L/28R – 10C/28C – 10R/28L at Chicago O’Hare).

Finally, the next time you’re learning spotting locations, or if you’re looking out the window of your aircraft and see those stretched, 63-foot numbers painted on the runway, keep in mind that if the number is greater than 18, subtract 18 to figure out the other side. If it’s 18 or less, add 18 to figure out the other side. A runway name will never be greater than 36: 360 degrees in a compass.