The line of longitude used as the origin in a system of coordinates is the Prime Meridian.

Here's why:

* Longitude: Longitude measures the angular distance, east or west, of a point on the Earth's surface from the Prime Meridian.

* Prime Meridian: This imaginary line runs from the North Pole to the South Pole and passes through Greenwich, England. It's the starting point for measuring longitude.

Think of the Prime Meridian as the "zero" line for longitude. All other longitudes are measured in degrees east or west of the Prime Meridian.