What Your Flight Number Really Means
Air travel can be a complicated process, and between packing, paperwork, layovers, and other complications, most people don't pay much attention to their flight numbers. However, these letter-number combinations each has their own unique meaning.
Each flight number starts with a two-character airline abbreviation. Many are obvious: AA for American Airlines, BA for British Airways, DL for Delta, etc. A few are less clear: Southwest Airlines is WN because SW already belonged to Air Namibia. JetBlue is B6 because JB belonged to Helijet International.
The numbers following these airline abbreviations can only be four digits long, ranging between 1 and 9999. The highest numbers are often reserved for charter flights, such as for a sports team, or ferry flights moving empty aircraft from one airport to another. Flight numbers must be different for flights operating at the same time. In addition to helping airlines keep track, they also help air traffic control monitor flight routes (via Blue Sky News).
How the numbers are chosen
Certain rules apply to each flight number in addition to length. Usually, flights heading east or northbound have even numbers, while flights heading west or southbound have odd numbers. Lower numbers are reserved for an airline's most popular or important routes (via Mental Floss). Some key routes for certain airlines are number 1, including AA1, American Airlines's flight from New York City's Kennedy International Airport to Los Angeles, a route in operation since 1939. UA1 is United Airlines's longest flight, from San Francisco to Singapore (via Blue Sky News).
Since there is a finite amount of numbers airlines can use, some conserve them by using the same flight number for inbound and outbound flights on the same route. For example, DL318 is Delta's flight from Atlanta to Myrtle Beach, S.C., and back (via Blue Sky News). Other airlines use one number lower or higher for a return flight, for example, U.S. Airways's flights 1208 and 1209 are between Philadelphia and St. Maarten. Flight numbers for a certain route rarely change except sometimes after a crash or other disaster (via Mental Floss).
Creative flight numbers
Whereas most flight numbers are assigned for practical reasons, sometimes airlines get creative and choose a number based on something to do with the city it's coming from or going to. Some examples include Alaska Airlines's flight from Seattle to Pittsburgh AS412, named for Pittsburgh's area code. American, U.S. Airways, and JetBlue all have flights between Boston and Philadelphia numbered 1776 for the year of American independence. Several airlines have flights to or from Indianapolis numbered 500 for the Indy500 race (via Blue Sky News and Travel Skills).
Other creative JetBlue flights are 66 from Albuquerque to Kennedy, named for Route 66, and 1600 from Washington's Reagan National Airport to Boston's Logan International Airport, numbered based on the address of the White House (via Travel Skills).
According to Simple Flying, only one route in the U.S. uses number 666, the "devil's number:" a Sun Country Airlines flight from Tucson, Ariz., to Minneapolis. RyanAir's daily flight from Dublin to Birmingham, England, is also flight 666 and is locally known as "the devil's flight" (via Travel Skills).