Typically, flights going eastbound or north are assigned even 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 Philadelphia to St. Maarten U.S. Air flight 1209 would take you down to the Caribbean (southbound) and flight 1208 would bring your back to the states (northbound)