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The system of longitude lines has the following characteristics:

 Lines of longitude run from the North Pole to the South Pole (top to bottom of the map) and are called meridians.

 As opposed to latitude, no two lines of longitude are parallel to each other. Rather, successive lines of longitude are about 70 miles apart at the equator, but from there they slowly converge until they come together at the two poles (see Figure 3-2).

 The prime meridian (Longitude 0°) divides the world into the Eastern Hemisphere and the Western Hemisphere.

 Starting from the prime meridian, every line (degree) of longitude is numbered consecutively to the east and to the west half way around the world. Because Earth is 360 degrees around, 180 degrees of longitude lie east and west of the prime meridian.

 Every line of longitude (except the prime meridian and the 180-degree line) is identified by a number from 1 to 179, and by the words East or West (or the letters E or W) to indicate its location east or west of the prime meridian. Thus, the line that is 20 degrees east of the prime meridian is referred to as Longitude 20° East. It would be misleading to call this line “Longitude 20” because some another line that is 20 degrees west of the prime meridian also could be called “Longitude 20.”

 Every line of longitude is a great circle — a line which, if continued around the world, would divide Earth equally in half.