Rail service fares in Greater London and the surrounding area are calculated in accordance with the London fare zones system managed by Transport for London. Within London all London Underground, National Rail, London Overground, TfL Rail and Docklands Light Railway stations are assigned to nine fare zones. Fare zone 1 covers the central area and fare zones 2, 3, 4, 5 and 6 form concentric rings around it. Some National Rail stations and all Transport for London served stations outside Greater London in the counties of Buckinghamshire, Essex, Hertfordshire and Surrey are either included in fare zones 4,5 or 6 or in extended zones beyond these. Transport for London fare zones are also known simply as zones or travelcard zones, referring to their use in calculating prices for the travelcard product. Before flat fares were introduced in 2004, fare zones were used on the London Buses network. London fare zones are also used for calculating the cost of single and return paper tickets, Oyster card pay-as-you-go fares and season tickets.