This depends on the airline. Some methods to the madness are: The low numbered flights are most often the premier, long haul routes of an airline out of their home city. For example American Airlines flight 1 has been flying coast to coast for over 60 years from their old headquarters city of New York. Formerly known as “The Sun Country Special” when it was DC-3 service, Flight 1 ended up at the old Lockheed Air Terminal in Burbank, after stops in Washington D.C.; Cincinnati, Ohio; Memphis, Tennessee; Dallas and El Paso, Texas; and Phoenix, Arizona along the way. Today AA flight 1 still goes coast to coast, but now it's non-stop service from JFK to LAX. (As are flights 3 & 5. And yes, AA flights 2 & 4 still go LAX to JFK). The same is true for other carriers, even those not flying long haul in the beginning like Southwest. Their original route system was Dallas, San Antonio & Houston. SW flight 1 still flies from their home of Dallas to Houston and flight 2 returns. Throughout the years, when flight times have changed or certain flights have been dropped, two, three or even four digit numbers have come in numbering service that was added later, often without much thought as to what routes got what flight numbers unless ceratain city pairs were assigned certain number blocks by the route planners. Interestingly, the number 8 has some connotation of luck and prosperity associated with it for the Chinese. Probably a reason why all United Airlines flight numberss to/from Asia start with that number. Also, airlines will often give flights odd or even numbers depending on whether they are going to or from a hub. AA numbers flights leaving hubs with odd numbers and returning service with even flight numbers. Certain groups of numbers are also assigned to an airlines commuter or codeshare flights so employees can quickly differentiate other carriers flights operating under their brand name. The rest are pretty much just computer generated assigned flight numbers.