You see them everywhere: On highway signs, plane tickets, and even humblebraggy Twitter updates. Some are obvious and some make no sense. But what do these three-letter airport codes really mean?

The sleek new site Airport Codes lets you bounce from airport to airport exploring the codes and their origins. Here are some of the more amazing, intriguing, and just plain strange stories behind those three little letters.

LAX, PDX, PHX: The reason so many airports, like Los Angeles, Portland and Phoenix have a code that ends in X is because up until the 1930s, airports only had two-letter codes. So LA became LAX, and so forth.

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SFO: The one exception to this rule is San Francisco, which wanted to use the O at the end of its name.

EWR: The Navy snapped up all the N codes, so Newark had to make do with the only available letters in its name.

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IAD: Dulles International Airport used to be DIA, but it was misspelled as DCA, another DC-area airport. So the code was changed to avoid confusion.

ORD: Chicago's major airport's code isn't a bad spelling of O'Hare. It used to be Orchard Field.

SUX: Poor Sioux City, Iowa tried to get its code changed, to no avail. SUX for them.

FAT: Sorry, Fresno Yosemite International Airport, you were originally named Fresno Air Terminal. This is why you're FAT.

YVR, YWG: In one of the only country-wide code examples, Canadian airports like Vancouver and Winnipeg all have codes start with Y. Why Y? No one knows.

YYZ, YUL: However, these two Canadian airports have codes that no one can explain. Toronto (YYZ) and Montreal (YUL) might be named for nearby railway stops and radio stations, respectively.

What are some of your favorites that we didn't mention? [Airport Codes via Co.Design]