Boeing customer codes have been used by Boeing Commercial Airplanes to identify the original customer of an aircraft since the Boeing 707. Boeing Commercial Airplanes, based in Renton, Washington, is a unit of The Boeing Company, consisting of the Seattle-based former Boeing Airplane Company (the civil airliner division), as well as the Long Beach-based Douglas Aircraft division of the former McDonnell Douglas Corporation. ... The Boeing 707 is a four-engine commercial passenger jet airliner developed by Boeing in the early 1950s. ...
Example would be a Boeing 747-400 ordered by British Airways would be a Boeing 747-436 (36 being the customer code). The codes do not change if the aircraft is subsequently sold as they reflect the original configuration of the aircraft.
The order of codes has not been sequential, as the first 707 was designated the 707-120 by Boeing, so the customer codes started at 21:
21 to 99 - First Sequence
01 to 19 - Second Sequence
0A to 9Z - Third Sequence
AA to ZZ - Fourth and current sequence
Airlines in italics are defunct. This is a list of defunct airlines. ...