Lines of longitude are imaginary lines drawn on a map running from the North Pole to the South Pole.

Lines of longitude are also measured in degrees like lines of latitude.

The prime Meridian or Greenwich Meridian is the major line of longitude.

Lines of longitude

The prime meridian is called so because it crosses a town in London called Greenwich.

All other meridians are numbered from it. The two known meridians are prime meridian and International Date line.

All the meridians East of Greenwich are numbered from 0 to 1800E of Greenwich while meridians west of Greenwich are numbered from 0 to 1800W of Greenwich meridian.
 

Uses of lines of longitude