This document discusses geographic coordinate systems. It explains that longitude and latitude grids allow every location on Earth to be identified by a set of numbers or letters. Parallels are lines of equal distance from the equator that run east to west, while meridians are lines of equal distance from the north and south poles that run north to south. The prime meridian at Greenwich, England is used as the base line for measuring longitude, with lines running 180 degrees east and west from it.