Something has been bugging me for a while, so I hope y'all have the answer...

I've come to the conclusion that Canada has made the decision to have the 1st letter of all its airports start with "Y"... but why?

Another thing... given the above, I understand YVR for VancouveR and YOW for OttaWa... but why YYZ for Toronto, YEG for Edmonton, and YUL for Montreal?