The following is an encore presentation of Everything Everywhere Daily.

If you've done your share of flying,

then you're probably familiar with the three-letter airport codes which identify every commercial airport in the world.

Airports like DFW, LGA, and H-O-U are pretty easy to figure out.

However, why is there an X in LAX, and how did Washington Dulles wind up with IAD?

And what's the deal with almost every airport code in Canada?

Learn more about airport codes and the weird logic behind them on this episode of

everything everywhere daily. The history of airport codes dates back to before airplanes were even invented.

The United States National Weather Service was created after the Civil War.

They would take weather observations at military bases around the country and would transmit the weather reports via telegraph to other bases.

To facilitate sending the reports over telegraph, codes were developed for American cities.

These two letter codes were

shorthand for telegraph operators.

When commercial aviation developed in the United States in the 1930s,

pilots began to use the National Weather Service codes to identify the cities they flew to.

However, there were two big problems.

First, not all US cities had a weather service code.

And second, there weren't enough two-letter combinations to cover all the cities,

especially if the system were to be used outside of the United States.

There are only 676 possible two-letter combinations.

To solve these limitations, a three-letter combinations. To solve these limitations a three-letter system was developed.

Assuming that every combination of letters was used, there are 17,556 possible three-letter

...