Time zones are an international convention held in place in all nation to aid in distinguishing different times between different cities. The first official time zones were established in 1881 at the International Meridian Conference. The Prime Meridian was set through Paris, France, and its time zone is known as PMT (Paris Mean Time) or TUC (Temps Universel Cordonné). All other time zones are named by their hourly offset from the Prime Meridian. Each time zone is 15°, although international and internal borders often cause certain areas to be added into another time zone.