Why do Canadian airport codes all begin with a Y?

There are two types of airport codes: 4-letter ICAO codes used for flight operations/ATC, and 3-letter IATA codes used for passenger handling. The ICAO codes consist of a two-letter country prefix and a two-letter airport code. If a country has a lot of airports it may have more than one prefix. For example, the US has 26 – KA to KZ. Canada has four, CU, CW, CY and CZ, with the overwhelming majority of airports that have scheduled service falling under CY. In most areas of the world ICAO codes and IATA codes bear no resemblance to each other. For example, LHR (Heathrow) is EGLL, AMS (Amsterdam) is EHAM, CDG Charles de Gaulle) is LFPG. However, in the US and Canada, the IATA codes are based on the ICAO codes with the first letter of the prefix dropped. Thus KJFK becomes JFK, KLAX is LAX, CYOW translates to YOW for Ottawa, and CYYZ is YYZ for Toronto. Don’t think that any “”Y”” code will be in Canada though – YUM is Yuma, AZ, and YOK is Yokohama, Japan.