Why do runways have numbers?
The short answer
The large numbers near the ends of a runway show which direction its centerline points. They help pilots and air traffic controllers identify the runway and the end being used. In a common system, the number is based on the runway’s direction from magnetic north, rounded to the nearest 10 degrees.
How to read the number
Imagine a compass starting at magnetic north: 0 degrees, or 360 degrees. Directions increase clockwise. The runway direction is rounded to the nearest 10 degrees, and the final zero is dropped. For example, a centerline pointing about 87 degrees from magnetic north rounds to 90 degrees, so its runway number is 09.
The same runway looks 180 degrees different when viewed from the other end. Runway 09 points roughly east; the opposite end is Runway 27, pointing roughly west. The numbers do not show a runway’s length or its place in a sequence. A runway pointing almost toward magnetic north is numbered 36.
Letters distinguish parallel runways
Large airports may have two or more runways pointing in the same direction. The numbers alone would not tell them apart, so a letter is added. L, R, and C mean left, right, and center, as seen by a pilot approaching the runway. For example, 09L and 09R point roughly east but identify different runways.
The number and the runway in use
A runway number marks a direction; it does not mean the runway is always used that way. Aircraft often take off and land on a runway that is nearly aligned with the wind, so the direction in use can change when the wind changes. Wind is not the only factor: runway conditions, traffic, and airport operations can also matter.
Once you know what the numbers mean, they look less like labels for places and more like a shared way to describe direction. A compass explains where magnetic north comes from, while the wind helps explain why aircraft may use one end of a runway instead of the other.