Some National Rail stations and almost all Transport for London served stations outside Greater London in the home counties of Buckinghamshire, Essex, Hertfordshire and Surrey are either included in fare zones 4, 5 or 6 or in extended zones beyond these.
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.
The purpose of creating zones was to simplify fares, in order to speed up the process of buying tickets.
The whole of Greater London was divided into bus zones where flat fares applied.
Inner zone 1 forms a roughly circular area and covers central London.
[1] In January 2016, the Oyster and contactless system was extended to Gatwick Airport in Crawley, West Sussex, and the stations down that line (Horley, Salfords, Earlswood, Redhill and Merstham).