Boarding passes show three-letter airport codes like LHR and JFK, while pilots use four-letter codes for the same airports. Both systems exist for good reasons.

IATA codes are three letters for travellers: tickets, boarding passes, and booking sites. They are memorable where possible, and historical where the obvious letters were taken.

ICAO codes are four letters for pilots and controllers, with a regional prefix: E for northern Europe, K for the US mainland. Heathrow is EGLL; JFK is KJFK.

Travellers mostly use IATA codes, but flight trackers and aviation news often show ICAO codes. Knowing both explains why the same airport sometimes appears under two different codes.