What are those numbers and letters for on the back of every plane’s fuselage?

Q: What are those numbers and letters for on the back of every plane’s fuselage?

That’s the ship’s registration, which like a car’s license plate is displayed on the exterior of every plane. All US registrations are prefixed with the letter N, followed by a sequence of numbers and, usually, letters. A United 747 registration: N184UA. Note the UA denotes United Airlines, but the letters don’t always follow this rule. Next time you fly, jot down the registration, and using my reference materials I can tell you all sorts of, um, fascinating and sundry information about your plane, including its date of construction.

Other countries do it differently, some using only letters, numbers or both. An Air France Concorde registration: F-BVFB. Often the prefix coding is intuitive, such as G for Great Britain, JA for Japan, and D for Germany. Others seem inexplicable, like Brazil’s PP, Belgium’s OO, and even our own N. If ever you notice really strange one, let me know and I can decipher it for you.

Registrations don’t always change when a plane is sold, and occasionally a jet’s numbers can give insight into its history. If you see an old 727 freighter with a registration ending in “AA,” chances are it once flew passengers in the colors of American Airlines. Planespotters use registrations to keep track of which aircraft they have and haven’t seen (or at least they used to, until being banished from airports after 9/11). In grade school I had notebooks full of them.

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This article is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004” by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.