the letter ''K'' was simply assigned to the contiguous US by ICAO, in order to have a system with unique identifiers for world-wide use, instead of trying to adapt local system to match. The IATA codes had been in use already and possible duplicates could not be excluded.

The "K" prefix for airports in the contiguous United States is part of the ICAO (International Civil Aviation Organization) four-letter coding system used for global flight planning and navigation. In this system, the first letter represents the region, and "K" was assigned to the continental U.S. (Alaska and Hawaii use "P" for Pacific). This convention originated from early 20th-century radio call signs; at the time international standards were being set, the U.S. already used "K" for western radio stations and "W" for eastern ones. For simplicity and global standardization, the ICAO adopted "K" for the entire mainland. Most large U.S. airports simply add a "K" to their three-letter IATA code (e.g., LAX becomes KLAX). This fourth letter is essential for digital flight databases and air traffic control computers to distinguish between thousands of airports worldwide. Smaller airstrips that do not see international traffic often omit the "K" and use a three-character FAA identifier instead, but for the global aviation network, "K" is the definitive marker of a U.S. mainland airport.