How Do Airports Get Their 3-letter Codes?

If you’ve ever booked a flight, you’re probably aware every airport in the world has a three-letter code. It’s not surprising airports would have such an identifier, since it’s easier to say and write than a full name. “Brevity on the radio is important, especially in busy air traffic locations,” former airline pilot Dan Bubb, a professor at the University of Nevada, Las Vegas, tells Travel + Leisure.

In many cases, those codes are fairly logical — JFK, for John F. Kennedy International Airport in New York, for instance, or MIA for Miami International Airport. But some are head-scratchers, like ORD for Chicago O’Hare International Airport and MSY for Louis Armstrong New Orleans International Airport. So, how in the world do all these airports get their codes? It all goes back to the early days of aviation.

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“Airport coding began in the 1930s,” says Bubb. Back then, pilots used two-letter codes for cities designated by the National Weather Service. “However, that system had its limits, which became apparent in the 1940s, when more airports appeared,” adds Bubb. Suddenly, the number of two-letter codes (a possible 676 combinations) wasn’t enough to meet demand. As such, airports were given three-letter codes (a possible 17,576 combinations).

Some airports were assigned three-letter codes to match their city names, such as MIA for Miami. Others had an X tacked onto their two-letter code, like LAX for Los Angeles, PHX for Phoenix, and PDX for Portland, Oregon. By the 1960s, it was clear an international organization needed to organize and standardize the process. Enter the International Air Transport Association (IATA).

IATA now has a hierarchy for establishing new airport codes around the world. “They try to make the codes as intuitive and parallel as they can to minimize confusion for pilots,” says Bubb. First, IATA goes to the city name, typically choosing the first three letters. Another top option is an acronym for the airport, like CDG for Paris Charles de Gaulle Airport.

But if a three-letter code is already taken by another airport — there can be no duplicates — IATA moves on to the next best option. In some cases, that’s the historical name for the airfield: ORD for Chicago O’Hare comes from Orchard Field Airport, the airport’s previous name. Other times, it references something else from the airport’s or city’s history. MSY (Louis Armstrong New Orleans International Airport), for example, stands for Moisant Stock Yards — the airport was built on the stockyard’s former grounds. (Interestingly, ​​the stockyards were named after aviator John Bevins Moisant, who died in a plane crash on that land.)

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There are also some quirks in the system. For instance, the U.S. Navy reserved all N codes, leaving some N cities with bizarre airport codes (here’s looking at you, EWR, Newark Liberty International Airport). And in Canada, most airport codes start with a Y — it’s a long story, but in the 1940s, Canadian airport codes either started with a Y for “yes” or W for “without,” referring to whether or not they had a weather station radio tower nearby.

There’s also an entirely separate airport code system managed by the International Civil Aviation Organization (ICAO). ICAO codes are four letters, and they also have a specific system: Airports within a given region all start with the same one or two letters. In the contiguous U.S., that letter is K, while Hawaii and Alaska use PH and PA, respectively. Generally speaking, IATA three-letter codes are more commonly used by passengers, whereas ICAO codes are typically employed by professionals, from pilots to air traffic controllers.

So, the next time you have a boarding pass in your hands (or on your phone), take a look at the airport codes, and remember there’s a rich history behind those not-so-random jumble of letters. And they’re one of the many, many reasons air travel is as efficient as it is today.