Old Smokey paints a very good picture. However the rules change for the North Atlantic. Levels are swapped throughout the North Atlantic Track Structure during different times of the day/night. Eastbound tracks are usually fl320-400 (0100-0800Z)within the tracks, and Westbound levels are usually 310-400 (1130-1900Z). Levels vary North or South of the tracks depending on time for 30w, and direction of flight during the East or Westbound flow.