LAX, yeah, but ORD? IAD?

From the department of how stuff works, an article about airport codes.

It's obviously much easier for pilots, controllers, travel agents, frequent flyers, computers and baggage handlers to say and write ORD than the O'Hare International Airport in Chicago, Illinois -- but how did this practice start, and why are some airport codes easy to understand (ABE and ZRH) while others seem to make absolutely no sense (ORD)?
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Some special interest groups successfully lobbied the government to obtain their own special letters. The Navy saved all the new 'N' codes. Naval aviators learn to fly at NPA in Pensacola, Florida and then dream of going to ''Top Gun'' in Miramar, California (NKX). The Federal Communications Committee set aside the 'W' and 'K' codes for radio stations east and west of the Mississippi respectively. 'Q' was designated for international telecommunications. 'Z' was reserved for special uses. The Canadians made off with all the remaining 'Y codes which helps explain YUL for Montreal, YYC for Calgary, etc. One of the special uses for 'Z' is identifying locations in cyberspace. What am I talking about? Well, an example is ZCX the computer address of the FAA's air traffic control headquarters central flow control facility. ZCX is not an airport but a command center just outside Washington D.C., that controls the airline traffic into major terminals.

Lacking both 'W' and 'N' Washington National has a code of DCA for District of Columbia Airport. The newer Dulles airport just outside D.C. was DIA (from Dulles International Airport); however, the DIA and DCA were easy to confuse, especially when hastily written in chalk on a baggage cart, scribbled on a tag or a handwritten air traffic control strip, so we are stuck with the backwards IAD. Now one of the rules of the game is ''the first and second letters or second and third letters of an identifier may not be duplicated with less than 200 nautical miles separation.''