We all know that flight numbers are important when it comes to checking airport gate monitors or flight statuses online, but beyond that, do we really pay attention to them? While they might seem trivial to some degree in today’s world of technological check-ins, the numbers aren’t entirely random, and they aren’t meaningless. In fact, you can presume a lot about a flight just by its number.

According to Patrick Smith, former airline pilot and author of Cockpit Confidential, a flight number is technically a combination of numbers and letters prefaced by the carrier’s two letter code—Delta is DL, United is UA, and American is AA, etc.

Typically, flights going eastbound or north are assigned even numbers, and those headed west or south get odd numbers (there are exceptions to this rule, though). Return flights between destinations are often assigned a number that is one higher/lower than the outbound flight. So, if you were flying roundtrip from Philadelphia to St. Maarten, U.S. Air flight 1209 would take you down to the Caribbean (southbound) and flight 1208 would bring you back to the States (northbound).

In general, the lower the number, the more “prestigious” the flight route is for that particular airline. One or two-digit numbers are typically assigned to popular routes—usually of the long-distance variety—such as United Flight 44 from Newark to London. If you find yourself on a flight with a low number, it’s a pretty safe bet that your flight is a regular moneymaker for the airline.

Flight numbers made up of four-digit sequences starting with 3 or higher are ordinarily an indication of a code-share flight. Think U.S. Airways Express where you are flown between destinations on behalf of U.S. Air. You might purchase your ticket through U.S. Air, but the plane and crew belong to a separate partner airline.

As frequent business flyers have no doubt noticed, flight numbers along a specific route can remain unchanged for years barring any sort of incident. This is a bit of a downer example, but American’s daily departure from Boston to Los Angeles had been flight AA11 for decades until the attacks of September 11, 2001. Out of respect, airlines are quick to change the flight number of a route after an incident, solidifying their place in history, which is why so many unfortunate disasters of the past can still be easily referred to by their flight numbers.

Because exceptions exist and airline policies differ, these trends are far from set in stone, and you can find contradictions in the above school of thought—sometimes directly, such as an odd-numbered flight flying east. But what can we say? Perfection has never been the airline industry’s cup of tea.