The L and R on runways stand for Left and Right. They distinguish between parallel runways that point in the same direction at an airport. When two or more runways run side by side, pilots and air traffic controllers need a way to tell them apart, so each one gets a letter suffix: L for the runway on the left, R for the one on the right, as seen from the approach direction. If there’s a third parallel runway in the middle, it gets the letter C for Center.
How Runway Numbers Work
Before the letters make sense, it helps to understand the numbers. Every runway is named after its magnetic compass heading, rounded to the nearest 10 degrees, with the last digit dropped. A runway pointing roughly east at 90 degrees becomes Runway 9. One pointing west at 270 degrees becomes Runway 27. The numbers always range from 01 to 36, covering the full 360-degree compass.
Every physical strip of pavement has two numbers because you can use it in both directions. The two numbers always differ by 18 (since the opposite direction is 180 degrees away). A runway designated 12 on one end is 30 on the other.
Why L and R Are Needed
At a small airport with a single runway, the number alone is enough. But busy airports often have two or more runways running parallel to handle more traffic. Since these parallel runways point in the same direction, they’d all share the same number without some extra identifier. That’s where L and R come in.
Take an airport with two parallel runways both aligned to a heading of 270 degrees. Both would be Runway 27, which is obviously a problem. So the one on the left (from the pilot’s perspective as they approach to land) becomes 27L, and the one on the right becomes 27R. From the opposite direction, the designations flip: what was 27L becomes 09R, and 27R becomes 09L. This makes intuitive sense because if you turn around 180 degrees, what was on your left is now on your right.
Airports With Three or More Parallel Runways
When an airport has three parallel runways, the middle one is designated C for Center. So you might see 25L, 25C, and 25R at a large hub airport. Things get more creative when there are four or more parallel runways. Since the system only has three letters (L, C, R), airports with four parallels typically offset one runway’s number by one. For example, Dallas/Fort Worth has parallel runways designated as 17L, 17C, 17R, and 18L/18R. The headings are nearly identical, but bumping the number by one degree avoids having four runways share the same designation. It’s a practical workaround that keeps every runway name unique.
How Pilots and Controllers Use These Designations
The L and R designations aren’t just painted on pavement. They’re a critical part of every radio exchange between pilots and air traffic control. When a controller clears an aircraft to land, they specify the full designation: “Cleared to land Runway 27 Left.” Pilots read it back the same way. At airports with parallel runways, controllers also issue traffic advisories that reference parallel operations, such as alerting a pilot to traffic landing on the parallel runway. The letter suffix removes any ambiguity about which strip of concrete a 200,000-pound aircraft should be aiming for.
The designations also appear on taxiway signs throughout the airport, on approach charts pilots review before landing, and on the runway surface itself in large white numbers and letters visible from the air.
Why Runway Numbers Sometimes Change
Because runway numbers are based on magnetic compass headings, and Earth’s magnetic field slowly shifts over time, runway designations occasionally need updating. The FAA requires runway numbers to stay within a few degrees of the actual magnetic heading. When the drift accumulates enough (roughly 5 degrees or more), the airport repaints the numbers and updates all associated signage and charts.
How often this happens depends on location. Magnetic shifts occur faster near the poles, so high-latitude airports adjust more frequently. Fairbanks International Airport in Alaska, for example, renamed runway 1L-19R to 2L-20R in 2009 and expects to change it again around 2033, roughly a 24-year cycle. Airports closer to the equator can go much longer without a change. When a runway number shifts, the L and R designations shift with it. A runway currently called 17L could eventually become 18L if magnetic north moves enough to push the heading past the rounding threshold.
Reading Runway Markings From the Air
If you’ve ever looked out the window during landing and noticed giant white numbers on the pavement, that’s the runway designation. The number appears at both ends of the runway (different numbers on each end, 18 apart), and the letter L, R, or C is painted just below the number. These markings are standardized worldwide under both FAA and international (ICAO) rules, so the system works the same whether you’re landing in Chicago, London, or Tokyo. A pilot trained anywhere in the world reads 09R the same way: Runway 9, the right one.