Airlines have a choice when filing a flight plan.
The North Atlantic Tracks have been established to organise traffic between Europe and North America due to the complicated meteorological conditions. A jetstream running from roughly Alaska to North Eastern Europe prevents aircraft from flying fixed routings, as strength and location of this jetstream varies during the year.
It is basically a matter of great circle routings. Roughly estimated, all air traffic from France, Belgium, Germany, Luxembourg, The Netherlands, Great Britain and Ireland use NAT
routings to destinations on the Eastern half of the USA and Canada. Like I just said, it is a matter of great circle routings.
Flights "outside" the opening hours of the NAT
tracks (Tracks are established twice a day, once by Gander, once by Shannon), or if a NAT
track is not suitable will file a random route. These are a bunch of coordinates put together in a flight plan. (For instance N55W010 N56W020 etc etc..)
[Edited 2011-06-24 04:56:27]