was opened at London Heathrow Airport. Exchange codes From 1922, the first three digits of a seven-digit subscriber numberin other words, the local exchange codeswere represented with letters by way of a mnemonic. Each three-character code corresponded to an exchange within the London telephone area. The subscriber numbers were written, for example, as "ABBey 1234" and "WIMbledon 1234" or "ABB 1234" and "WIM 1234". By 1965, there were 350 local exchanges in London and the number of permutations that could be used for exchange names had been exhausted. With the change to all-figure dialling in 1966, the system of mnemonics was withdrawn and the three-digit local exchange codes of many subscribers were altered. The old codes continued to work in parallel with new codes until January 1970, when the "ANN: All-figure Numbers Now" advertising campaign prompted callers to use only the new codes. The transition to all-figure dialling allowed the codes to be grouped into eight sectors; all exchanges within of the centre of London formed the Central sector and the other sectors radiated from it. The first three digits of a subscriber number continued to indicate the exchange to which the number belonged; for example, "222 1234" was in Westminster (Central Sector) and "946 1234" in Wimbledon (South West Sector). More than one local exchange code was usually overlaid for each area, so all Wimbledon numbers did not necessarily begin "946". Subscriber numbers changed to eight digits in 2000 when an additional 7 or 8 was added to each local exchange code (for example, "7222 1234" in Westminster and "8946 1234" in Wimbledon). STD code The STD code 01 was assigned to the London telephone area on 6 April 1959 as part of preparations for subscriber trunk dialling. For the next thirty years, "01" became synonymous with the capital. Until May 1990, the 01 code covered the same area as the current 020 code and had capacity for fewer than 10 million telephone numbers. In May 1990, the 01 code was abandoned and the area divided between 071which covered exchanges in the Central sectorand 081, which covered all other sectors. Exceptionally, two exchanges in the East sector covering the London Docklands redevelopment area were assigned the 071 code. The anticipation that the code associated with central districts would be more prestigious than the other associated with the outer suburbs was used as a plot device in the Essex-based TV