The prime meridian is the zero (0) line of longitude. The zero line of latitude is a given in the form of the equator, but for longitude the line was agreed on at an international conference at Washington DC in 1884. (The conference chose a line that had widely accepted internationally). The matter was of particular interest to the U.S. where the present time zones had no official standing and about 2,000 (yes, two thousand) local times were in use.

The zero longitude line runs through the Royal Observatory in Greenwich in southeast London. It is marked on the ground in brass for about 200 yards.