When to use it
Use it when locating a point on a map to remember that longitude lines are the vertical ones that measure distance east or west of the prime meridian.
Example
The prime meridian sits at 0° longitude and runs long from the North Pole to the South Pole, passing through Greenwich, England.
Common mistake
Reading longitude values as up-down position. Longitude measures east-west location, not north-south position, which is latitude.
