Airport ABCs: An Explanation of Airport Identifier Codes

“From ABE (Allentown/Bethlehem/Easton, Pennsylvania) to ZRH (Zurich, Switzerland), airports around the world are universally known by a unique three-letter code: the ‘Location Identifier’ in aviation-speak. 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 ó 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)?”