On the global grid, the prime meridian is at

The Prime Meridian is a line running between the two Earth's Poles that divides the Earth into two Hemispheres: the Eastern to the right of it (as seen when the North is up) and Western to the left. It runs through the Uk and Greenwich observatory, France, Spain and West Africa.

It's also at 0 Degrees Longitude, (so 0 degrees East or West)