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Runways are numbered based on their direction and their position at the airport. The numbers range from 1-36, with each number standing for a certain direction on the compass rose. Each end of the runway has the number relating to the direction 180 degrees away from the other end. A number of 18/36 on either side of the runway would be a north-south runway, while a 9/27 runway would be east-west. The letters L, C, and R stand for left, center, and right, and are added to the numbers, for example 10L/28R and 10R/28L. The numbers help pilots tell apart two or three parallel runways at the same airport.