Every runway number is a compass heading divided by 10. Runway 27 points west — 270 degrees. Runway 09 points east — 090 degrees. Turn around and land from the other direction, and the number changes by 18.
That's it. The entire system in one sentence.
But the simplicity hides a planet-sized engineering constraint.
Runways are not numbered by choice. They are numbered by magnetic north — and magnetic north moves. The Earth's magnetic poles drift roughly 55 kilometres per year as molten iron churns beneath the crust. When the drift accumulates enough to shift a runway's magnetic heading into the next 10-degree bracket, the runway gets renumbered.
It happens more often than you'd think.
In 2009, London Stansted renumbered Runway 23/05 to 22/04. The physical strip didn't move. The planet's magnetic field did. The paint crew came out, restriped the numbers, updated every chart, briefed every pilot, and reprogrammed every navigation database.
Fairbanks, Alaska has renumbered twice since 2000. Near the magnetic pole, the drift is fastest. A runway that pointed at 020 magnetic a decade ago may now point at 018 — enough to cross the threshold and trigger a redesignation.
The L, C, R suffixes solve a different problem. When an airport has parallel runways pointing the same direction, they share the same number. Los Angeles has four parallel runways: 24L, 24R, 25L, 25R. Left, right, left, right — assigned from the pilot's perspective on approach.
Chicago O'Hare has eight runways across four orientations. The numbering grid reads like a coordinate system: 9L/27R, 9R/27L, 10L/28R, 10C/28C, 10R/28L, 14L/32R, 14R/32L, 15/33.
Every number is a direction. Every direction is magnetic. Every magnetic heading is temporary.
The runway is fixed in concrete. The number painted on it is fixed to a field that never stops moving.
Your flight landed on a compass reading. That reading has an expiration date.