Hi, here's a quote and the website the quote came from that state the answer to your question!
"Longitude measures the number of degrees EAST or WEST of the Prime Meridian which runs through Greenwich, England. The Prime Meridian has a longitude of 0 . Lines of longitude (meridians) running north-south around the globe measure distances EAST and WEST of the Prime Meridian."
www.jsu.edu/depart/geography/mhill/phygeogone/latlngpr.html
*** DISCLAIMER *** This is not my website or quote, I've cited and quoted the website properly.