The prime meridian is internationally recognized as the longitude of the Royal Observatory at Greenwich near London, England, because of the influence of the United Kingdom in the 1800s. This location was selected at an international conference in 1884 by a vote of 22 out of 25 countries.
The prime meridian is the global longitude line at zero degrees in the same way that the equator is the global latitude line at zero degrees. Before 1884, many countries had their own local prime meridians that they used to establish internal coordinate systems for mapping and navigation. However, since the United Kingdom was a major navigational power with colonies around the world, its maps were spread to many countries that eventually adopted the Greenwich meridian as their standard.