While the routes change daily, they maintain a series of entrance and exit waypoints which link into the airspace system of North America and Europe. Each route is uniquely identified by a letter of the alphabet. Westbound tracks (valid from 11:30 UTC to 19:00 UTC at 30W) are indicated by the letters A,B,C,D etc. (as far as M if necessary, omitting I), where A is the northernmost track, and eastbound tracks (valid from 01:00 UTC to 08:00 UTC at 30W) are indicated by the letters Z,Y,X,W etc. (as far as N if necessary, omitting O), where Z is the southernmost track. Waypoints on the route are identified by named waypoints (or "fixes") and by the crossing of degrees of latitude and longitude (such as "54/40", indicating 54°N latitude, 40°W longitude).