London fare zones

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.

Before the introduction of fare zones, tickets for rail travel in Greater London were purchased on a 'point-to-point' basis between two stations, either as a single, return or season ticket; and were priced according to distance travelled. During the early 1980s the London Transport Executive of the Greater London Council made a series of revisions to fares which introduced the fare zones. The purpose of creating zones was to simplify fares, in order to speed up the process of buying tickets. On buses this became necessary as the use of a conductor was being eliminated in favour of the driver selling tickets, which was having an impact on the time it took passengers to board the bus and therefore on journey times.

...
Wikipedia