Why airports have two codes, how each is built, and how to look up an airport with its runways and nearby alternatives.
Airports usually have two codes: a three-letter IATA code used by airlines and passengers (LHR, JFK) and a four-letter ICAO code used by air-traffic control (EGLL, KJFK).
IATA codes are assigned for commercial branding and are not systematic — some are obvious (LHR for Heathrow), others are historical. ICAO codes are regional: the first letter identifies the region (K for the contiguous US, E for northern Europe), followed by a country and airport identifier.
The free Airport Codes directory covers 10,181 airports from the OurAirports dataset: IATA and ICAO codes, type, elevation, runways, coordinates, city and country, and the nearest airports with distances.
Cheap fares often involve a secondary airport an hour away. Each airport page lists the nearest airports with distances, so the trade-off is visible before booking.