Aviation is controlled by an agency of the United States' government known as the Federal Aviation Administration or the FAA. This agency mandates identification standards for airport layout that is meant to assist pilots in easily recognizing runways from the air and to taxi safely from the runway to the gate. From runway numbers and painted stripes to airport and runway lights and signs, the FAA regulates the National Airspace System.
In navigation and surveying, all measurement of direction is performed by using the numbers of a compass. A compass is a 360° circle where 0/360° is North, 90° is East, 180° is South, and 270° is West. Runways are laid out according to the numbers on a compass. A runway's compass direction is indicated by a large number painted at the end of each runway. Preceding that number are 8 white stripes. Following that number by 500 feet is the "touchdown zone" which is identified by 6 white stripes. A runway's number is not written in degrees, but is given a shorthand format. For example, a runway with a marking of "14" is actually close to (if not a direct heading of) 140 degrees. This is a southeast compass heading. A runway with a marking of "31" has a compass heading of 310 degrees, that is, a northwest direction. For simplicity, the FAA rounds off the precise heading to the nearest tens. For example, runway 7 might have a precise heading of 68 degrees, but is rounded off to 70 degrees.
Each runway has a different number on each end. Look at the diagram below. One end of the runway is facing due west while the other end of the runway is facing due east. The compass direction for due west is 270 degrees ("27"). The compass direction for due east is 90 degrees ("9"). All runways follow this directional layout. This runway would be referred to as "Runway 9-27" because of its east-west orientation.