A flight number is a unique alphanumeric code assigned to a specific airline route and schedule, such as AA100 or EK201. An example of a classic flagship service is BA1, which historically represented British Airways' premier flight from London to New York. The code consists of a two-letter (or one letter, one number) IATA airline designator—like "DL" for Delta, "LH" for Lufthansa, or "SQ" for Singapore Airlines—followed by a numerical string of up to four digits. Traditionally, eastbound and northbound flights are assigned even numbers (e.g., AA2), while westbound and southbound flights use odd numbers (e.g., AA1). For daily services with multiple departures, the numbers usually increase throughout the day. It is important to distinguish the flight number from the tail number (the plane's actual "license plate") or the PNR/Booking Reference; the flight number tells you where and when the plane is going, but not which specific physical aircraft will be performing the journey on that day.