The Convention on International Civil Aviation (the Chicago Convention) was signed in Chicago on 7 December 1944. Pending ratification of the Convention, the Provisional International Civil Aviation Organization (PICAO) was established in 1945. ICAO formally came into existence on 4 April 1947 as a specialized agency responsible for promoting the safe and orderly development of international civil aviation.
In 1947 ICAO introduced the two-letter airline designators and telephony designators for use in Flight plans, AFTN messages and Air Traffic Control (ATC) identification together with a telephony (callsign) designator.
In the early 1950s IATA adopted the ICAO two-letter designators for its member airlines for use in passenger and cargo reservations, ticketing, baggage handling and published timetables.
By the late 1960s the U.S. Federal Aviation Administration (FAA) had developed a separate system of three-letter operator contractions for domestic use in flight plans, ATC and aeronautical communications. These designators appeared in successive editions of FAA Contractions Handbooks (FAA Order 7340 series) and remained a U.S.-only system until replaced by the ICAO three-letter designators in October 1987.
In November 1981 the ICAO Council Working Paper C-WP/7342 proposed replacing the two-letter airline designator system with a three-letter system.
July 1987 ICAO published the first edition of Document 8585 Supplement containing the new three-letter designators (3LD) and associated telephony designators (callsigns).
The ICAO three-letter designators became effective on 25 October 1987, coinciding with the commencement of the 1987/88 IATA Northern Winter Schedule.
From 25 October 1987, ICAO exclusively used the new three-letter designator system for operational purposes, while IATA retained and continued to administer the existing two-character airline designator system for commercial applications. Since then, the ICAO and IATA airline designator systems have evolved independently.