Did you know that runways have names? Not names like 'Bob' or 'Polly': instead, they have numbers for names - and not just any number, only numbers from 01 to 36. Occasionally, some runways have a letter at the end of their name (more about that later).
A runway designation consists of two numbers each of two digits, one number being the reciprocal of the other. (This use of the term 'reciprocal' applies to navigation and compasses. It means the two numbers differ by 180°. If you prefer, think of it as the complement or modulus of the heading.)
One number is formed by rounding the compass bearing of one end of the runway up or down to the nearest 10° and dropping the last digit; if this results in a single digit, add a zero to the left of it. The other number is the reciprocal of the first number (see the table of Reciprocal Runway Numbers below). If a runway is aligned north-south, then it is 18/36, not 00/18. The lower number is always listed first.
When pilots and air traffic controllers refer to a runway, they use only the number that applies to the end the pilot will be landing on. Thus if the pilot is landing on Runway 09/27 heading to the east, they are using Runway 09, not Runway 27.
If the compass heading of a runway is 122° you would round it down to 120 and drop the last digit, leaving you with 12. Thus it is called Runway 12/30.
If the compass heading of a runway is 37°, you would round it up to 40 and drop the last digit, leaving you with 4. Since this is a single digit, you add a zero to the beginning, giving you 04. Thus it is called Runway 04/22.
Reciprocal Runway Numbers
|North/East end||South/West end|
So, About Those Letters...
Some airports have multiple runways that are parallel to each other. Obviously, you can't give such runways identical names, or you'll have very confused pilots1. Such runway numbers are followed by L, R or C (for Left, Right or Centre), respectively.
Airports that are in very northerly regions, such as northern Canada or Alaska, have a T after the runway number. This indicates to pilots that the number is based on the true bearing, rather than a magnetic bearing. True bearings are used because the magnetic north pole is not stationary, thus using a magnetic bearing causes problems. Also, compass readings become unpredictable when you're close to the magnetic north (or south) pole.