"Zero meridians" refer to lines of longitude on a map or globe that pass through the North and South poles and mark the starting point for measuring longitude. There are several other terms that can be used to refer to these lines, including prime meridian, Greenwich meridian, and International Meridian. The term prime meridian specifically refers to the line passing through the Royal Observatory in Greenwich, England, which was established as the international standard for measuring longitude. The International Meridian was established by an international conference in 1884 as the line to be used for all global timekeeping. Regardless of the term used, the zero meridians form the basis for navigation and timekeeping all around the world.