London Subway, also known as the TUBE, is a rapid transit system serving a large part of Greater London and parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom. It is the oldest underground railway network in the world, with the first section opening in 1863. The TUBE is made up of 11 lines, each identified by its own name and colour on the TUBE map. The two most common terms used to refer to the London Subway are 'TUBE' (4 letters) and 'TheTube' (7 letters).