Prime Meridian

The line of longitude that passes through the Royal Naval Observatory in Greenwich, England.  All other longitudes are measured from this line.

For example, 90 degrees West longitude lies 90 degrees (one quarter of the way around the earth) West of the Prime Meridian. Ninety degrees East longitude falls 90 degrees to the East of the Prime Meridian.

The Prime Meridian is the only line of longitude that has no East or West designation. The line falling exactly opposite the Prime Meridian, on the other side of the world, can be specified as either 180 degrees West longitude, or 180 degrees East longitude. This lines falls near the International Date Line.