Geographic grids are intersecting lines drawn on maps and globes. They are used to help locate and describe specific areas. The parallel lines of latitude are horizontal, running from east to west. Unlike lines of latitude, which become progressively shorter as they near the poles, all of the lines of longitude are the same length.
Longitudinal lines run from north to south, each individual vertical line intersecting both poles.
The prime meridian is an arbitrarily selected line from which all of the others are measured.