From Wikipedia:
Runways are named by a number between 01 and 36, which is generally the magnetic azimuth of the runway's heading in decadegrees. This heading differs from true north by the local magnetic declination. A runway numbered 09 points east (90°), runway 18 is south (180°), runway 27 points west (270°) and runway 36 points to the north (360° rather than 0°). When taking off from or landing on runway 09, a plane would be heading 90° (east).
A runway can normally be used in both directions, and is named for each direction separately: e.g., "runway 33" in one direction is "runway 15" when used in the other. The two numbers usually differ by 18 (= 180°).
If there is more than one runway pointing in the same direction (parallel runways), each runway is identified by appending Left (L), Center (C) and Right (R) to the number to identify its position (when facing its direction) — for example, Runways One Five Left (15L), One Five Center (15C), and One Five Right (15R). Runway Zero Three Left (03L) becomes Runway Two One Right (21R) when used in the opposite direction (derived from adding 18 to the original number for the 180° difference when approaching from the opposite direction). In some countries, if parallel runways are too close to each other, regulations mandate that only one runway may be used at a time under certain conditions (usually adverse weather).
At large airports with four or more parallel runways (for example, at Los Angeles, Detroit Metropolitan Wayne County, Hartsfield-Jackson Atlanta, Denver, Dallas-Fort Worth and Orlando) some runway identifiers are shifted by 10 degrees to avoid the ambiguity that would result with more than three parallel runways. For example, in Los Angeles, this system results in runways 6L, 6R, 7L, and 7R, even though all four runways are actually parallel at approximately 69 degrees. At Dallas/Fort Worth International Airport, there are five parallel runways, named 17L, 17C, 17R, 18L, and 18R, all oriented at a heading of 175.4 degrees. Occasionally, an airport with only 3 parallel runways may use different runway identifiers, such as when a third parallel runway was opened at Phoenix Sky Harbor International Airportin 2000 to the south of existing 8R/26L — rather than confusingly becoming the "new" 8R/26L it was instead designated 7R/25L, with the former 8R/26L becoming 7L/25R and 8L/26R becoming 8/26.
For clarity in radio communications, each digit in the runway name is pronounced individually: runway three six, runway one four, etc. (instead of "thirty-six" or "fourteen"). A leading zero, for example in "runway zero six" or "runway zero one left", is included for all ICAO and some U.S. military airports (such as Edwards Air Force Base). However, most U.S. civil aviation airports drop the leading zero as required by FAA regulation. This also includes some military airfields such as Cairns Army Airfield. This American anomaly may lead to inconsistencies in conversations between American pilots and controllers in other countries. It is very common in a country such as Canada for a controller to clear an incoming American aircraft to, for example, runway 04, and the pilot read back the clearance as runway 4. In flight simulation programs those of American origin might apply U.S. usage to airports around the world. For example, runway 05 at Halifax will appear on the program as the single digit 5 rather than 05
Runway designations change over time because the magnetic poles slowly drift on the Earth's surface and the magnetic bearing will change. Depending on the airport location and how much drift takes place, it may be necessary over time to change the runway designation. As runways are designated with headings rounded to the nearest 10 degrees, this will affect some runways more than others. For example, if the magnetic heading of a runway is 233 degrees, it would be designated Runway 23. If the magnetic heading changed downwards by 5 degrees to 228, the Runway would still be Runway 23. If on the other hand the original magnetic heading was 226 (Runway 23), and the heading decreased by only 2 degrees to 224, the runway should become Runway 22. Because the drift itself is quite slow, runway designation changes are uncommon, and not welcomed, as they require an accompanying change in aeronautical charts and descriptive documents. When runway designations do change, especially at major airports, it is often changed at night as taxiway signs need to be changed and the huge numbers at each end of the runway need to be repainted to the new runway designators. In July 2009 for example, London Stansted Airport in the United Kingdom changed its runway designations from 05/23 to 04/22 during the night.
For fixed-wing aircraft it is advantageous to perform takeoffs and landings into the wind to reduce takeoff or landing roll and reduce the ground speed needed to attain flying speed. Larger airports usually have several runways in different directions, so that one can be selected that is most nearly aligned with the wind. Airports with one runway are often constructed to be aligned with the prevailing wind. Compiling a wind rose is in fact one of the preliminary steps taken in constructing airport runways. Note that wind direction is given as the direction the wind is coming from: a plane taking off from runway 09 would be facing east, directly into an "east wind" blowing from 090 degrees.
Runway dimensions vary from as small as 245 m (804 ft) long and 8 m (26 ft) wide in smaller general aviation airports, to 5,500 m (18,045 ft) long and 80 m (262 ft) wide at large international airports built to accommodate the largest jets, to the huge 11,917 m × 274 m (39,098 ft × 899 ft) lake bed runway 17/35 at Edwards Air Force Base in California – developed as a landing site for the Space Shuttle.
Takeoff and landing distances available are given using one of the following terms:
TORA Takeoff Run Available – The length of runway declared available and suitable for the ground run of an airplane taking off. TODA Takeoff Distance Available – The length of the takeoff run available plus the length of the clearway, if clearway is provided. ASDA Accelerate-Stop Distance Available – The length of the takeoff run available plus the length of the stopway, if stopway is provided. LDA Landing Distance Available – The length of runway that is declared available and suitable for the ground run of an airplane landing. EMDA Emergency Distance Available – LDA (or TORA) plus a stopway.There exist standards for runway markings.
The runway thresholds are markings across the runway that denote the beginning and end of the designated space for landing and takeoff under non-emergency conditions. The runway safety area is the cleared, smoothed and graded area around the paved runway. It is kept free from any obstacles that might impede flight or ground roll of aircraft. The runway is the surface from threshold to threshold, which typically features threshold markings, numbers, and centerlines, but not overrun areas at both ends. Blast pads, also known as overrun areas or stopways, are often constructed just before the start of a runway where jet blast produced by large planes during the takeoff roll could otherwise erode the ground and eventually damage the runway. Overrun areas are also constructed at the end of runways as emergency space to slowly stop planes that overrun the runway on a landing gone wrong, or to slowly stop a plane on a rejected takeoff or a takeoff gone wrong. Blast pads are often not as strong as the main paved surface of the runway and are marked with yellow chevrons. Planes are not allowed to taxi, take off or land on blast pads, except in an emergency. Displaced thresholds may be used for taxiing, takeoff, and landing rollout, but not for touchdown. A displaced threshold often exists because obstacles just before the runway, runway strength, or noise restrictions may make the beginning section of runway unsuitable for landings.It is marked with white paint arrows that lead up to the beginning of the landing portion of the runway.