The
vertical longitude lines are
called "meridians". Think of these
as hula hoops cut in half, attached from one Pole to the other.
The longitude line numbers measure how
far east or west of the Prime Meridian a place is.
This Prime Meridian line runs vertically, right over Greenwich,
England, from the North Pole to the South Pole. It is numbered
as 0 degrees longitude, and numbers increase the further away
you move, up to 180 degrees. Longitude locations
are given as __ degrees east or __ degrees west.