The Prime Meridian passes through Greenwich, England. The original Royal Observatory, today's Flamsteed House, commemorates the establishment of the Prime Meridian with a museum and official denotations on the ground and, at night, in the sky of the line itself.
The Prime Meridian, as established by international treaty in 1884, is the official line marking 0 degrees longitude. The location of the Prime Meridian establishes the worldwide navigational starting point as well as dividing time zones. Greenwich Mean Time sets the convention for all global time measurements.
Prior to the establishment of the Prime Meridian, countries set their own navigational standards heedless of what their neighboring nations were using. For instance, China at one point used Beijing as their first meridian.