Los Angeles, Phoenix, Portland. Three cities whose airport codes share a letter that has nothing to do with their names. The X in LAX, PHX, and PDX does not stand for anything. It is packing material, added in the 1940s when aviation outgrew the two-letter codes it started with, and it stuck because nobody saw a reason to change it.
In the early days of commercial flight, airports were identified by the two-letter weather station codes already in use by the National Weather Service. Los Angeles was LA. Portland was PD. That worked while the system was small. Then aviation expanded fast in the 1930s and 1940s, international airports multiplied, and two letters could not keep up. IATA moved everything to three letters, and the airports with existing two-letter designations needed a third character.
Rather than inventing new combinations and retraining every pilot, dispatcher, and ticket agent, IATA simply bolted an X onto the end of the familiar codes. LA became LAX. PD became PDX. Phoenix got the same treatment and became PHX. The goal was continuity: the code still started with the letters people knew, and the X was understood to mean nothing. It was the path of least confusion, and seventy-plus years later the path is still there.
The X is not always a leftover
Not every X on a baggage tag comes from that 1940s upgrade. Dubai's airport is DXB, and Dubai never had a two-letter code to extend. The problem there was that DUB was already taken, by Dublin, Ireland. So DXB took the D, filled the middle with the placeholder X, and finished with the B. The X did the same job it did in 1940, filling space, but for a different reason: a collision rather than an expansion.
San Francisco went its own way. SFO ends with O not because of any rule, but because the city apparently wanted the O at the end of its name. It is the exception that shows how loose the whole system was. Nobody had written a specification for what the third letter should mean. That is the real story here: these codes were assigned by committee, accident, and habit, not by a schema.
Newark is my favorite related case, because it shows the same improvisation one letter over. Newark could not take NEW or NEW-adjacent codes starting with N, because the US Navy had reserved every code beginning with N. So EWR was built from the remaining letters of the city's name. The system had rules about who got which letters, but no rule about making them spell anything sensible.
So the next time a trivia night asks what the X stands for, the answer is nothing, and the reason is mundane in the best way. A two-letter system became a three-letter system, an X filled the gap, and billions of passengers learned it as if it had always been there. The codes are fossils of a decision made for convenience in the 1940s, still sitting on every departure board.
Frequently asked questions
What does the X in LAX stand for?
Nothing. It is a filler letter added in the 1940s when airport codes expanded from two letters to three. Los Angeles was LA under the old weather station system, and IATA added X to make LAX rather than inventing a new combination.
Why do so many airport codes end in X?
The biggest cluster, LAX, PDX, PHX, comes from the 1940s changeover from two-letter National Weather Service codes to three-letter IATA codes. IATA appended X to the existing two-letter designations so pilots and passengers would still recognize them.
Does the X in DXB mean something different?
Yes, slightly. Dubai's DXB got its X because DUB was already taken by Dublin, Ireland. The X still works as a filler, but the reason was a code collision, not the 1940s expansion.
Why is San Francisco SFO instead of SFX?
San Francisco simply preferred the O from its own name. The third letter was never governed by a strict rule, which is why SFO breaks the X pattern that LAX and PDX follow.
Why is Newark EWR instead of something starting with N?
The US Navy reserved all airport codes beginning with N, so Newark had to build its code from the remaining letters of its name: E, W, R.
Search the full US airport code database
Sources: The Points Guy, airport code history; National Geographic, decoding airport codes; Executive Traveller, what airport codes mean. Figures verified October 2026.