Most airports in the US have a three-letter International Air Transport Association (IATA }code, which is an identifier used to refer to the airport in online booking and consumer-facing operations. These codes don't often end in I (for International) or A (for Airport) because most codes would be entitled to end that way and it would leave us fewer codes to use [see below for some exceptions]. Instead, a system of abbreviations similar to postal abbreviations for US states has been developed, where the final letter of a n I suspect the O in SFO comes from the final letter of 'San Francisco'. .. letters throughout the airport name are used in the abbreviation. One example is EWR for Newark.
To have more options, the International Air Transport Association (IATA) required airports to have three letters instead of two, increasing the possible combinations from 650 to 15,600. Some airports added an X onto the end (LAX) SF decided to use O for the last letter in San Francisco.
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