Montréal-Trudeau International Airport, in Montreal, Canada.

What is the difference between YUL and CYUL?

YUL is the IATA code, used commercially — tickets, timetables, luggage tags. CYUL is the ICAO code, used operationally by pilots and air traffic control, and assigned geographically by region.

Is YUL the same as YMX?

No. YMX is mirabel, Montreal's abandoned second international airport. Cargo only since 2004.

What airport is YUL?

YUL is

Montréal-Trudeau International Airport

Montreal, Canada

Why the code is YUL

Y prefix plus UL, from the radio station at Kirkland, the district the airport was built in.

This is the explanation in general circulation and it is very likely right, but no primary document has been published that settles it. Treated as an account rather than a fact.

The airport

IATA code: YUL
ICAO code: CYUL
Airport: Montréal-Trudeau International Airport
City: Montreal, Canada
Code origin: The usual account, unverified

UL bears no relation to Montreal, Dorval or Trudeau, which is characteristic of Canadian codes. Montreal briefly had a second international airport, Mirabel, built for the 1976 Olympics and so far from the city that airlines abandoned it; it now handles only cargo.

Codes people book by mistake

Code	What it actually is
YMX	Mirabel, Montreal's abandoned second international airport. Cargo only since 2004.

The other code, CYUL

Every airport has two codes. The three-letter IATA code, YUL, is the one on your boarding pass and your luggage tag, and it is assigned for commercial use. The four-letter ICAO code, CYUL, is the one pilots and controllers use, and it is assigned geographically: the first letter or two identify the region, so every airport in the same part of the world shares a prefix. That is why CYUL looks nothing like YUL.

Common questions

What airport is YUL?: Montréal-Trudeau International Airport, in Montreal, Canada.
Why is Montreal's airport called YUL?: Y prefix plus UL, from the radio station at Kirkland, the district the airport was built in.
What is the difference between YUL and CYUL?: YUL is the IATA code, used commercially — tickets, timetables, luggage tags. CYUL is the ICAO code, used operationally by pilots and air traffic control, and assigned geographically by region.
Is YUL the same as YMX?: No. YMX is mirabel, Montreal's abandoned second international airport. Cargo only since 2004.

Canada Capital, area, population and currency
The flag of Canada Colours, symbols and lookalikes
Airport codes in Canada 7 here
What airport is YYZ? Toronto Pearson International Airport, Toronto
What airport is YVR? Vancouver International Airport, Vancouver
What airport is YYC? Calgary International Airport, Calgary
What airport is YEG? Edmonton International Airport, Edmonton
What airport is YOW? Ottawa Macdonald-Cartier International Airport, Ottawa
What airport is YHZ? Halifax Stanfield International Airport, Halifax

Sources

OurAirports — OurAirports (Public domain)

How these figures are compiled and checked