Longitude is a measure of a point's distance east or west of a standard reference point, which was established by international agreement: Greenwich, England. The lines of longitude are great circles that pass through both poles, and are perpendicular to the equator. These lines are frequently referred to as "meridians" of longitude. A longitude measure consists of a number from zero (on the meridian of Greenwich) to 180 (on the International Date Line) and a direction (east or west of Greenwich).