The two main longitudes are the Prime Meridian and the 180° meridian. The two longitudes divide the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere. Therefore, the longitude of a place is followed by the letter E for the east and W for the west. The 180° East and 180° West meridians are on the same line.

Further Reading: 

• GLOBE: LATITUDES AND LONGITUDES
• Geography Notes